Text Book Back Questions and Answers - Chapter 7 Atoms and Molecules 10th Science Guide Samacheer Kalvi Solutions - SaraNextGen [2024-2025]

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Chapter 7 - Atoms and Molecules - 10th Science Guide Samacheer Kalvi Solutions

I. Calculation of molar mass:
Question 1.
Calculate the gram molar mass of the following.
(i) $\mathrm{H}_{2} \mathrm{O}$
(ii) $\mathrm{CO}_{2}$
(iii) $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}$
Solution:
(i) $\mathrm{H}_{2} \mathrm{O}$
Atomic masses of $\mathrm{H}=1, \mathrm{O}=16$
Gram molar mass of $\mathrm{H}_{2} \mathrm{O}=(1 \times 2)+(16 \times 1)=2+16$
Gram molar mass of $\mathrm{H}_{2} \mathrm{O}=18 \mathrm{~g}$.
(ii) $\mathrm{CO}_{2}$
Atomic masses of $\mathrm{C}=12, \mathrm{O}=16$
Gram molar mass of $\mathrm{CO}_{2}=(12 \times 1)+(16 \times 2)=12+32$
Gram molar mass of $\mathrm{CO}_{2}=44 \mathrm{~g}$.
(iii) $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}$
Atomic masses of $\mathrm{Ca}=40, \mathrm{P}=30, \mathrm{O}=16$.
Gram molar mass of $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}=(40 \times 3)+[30+(16 \times 4)] \times 2$ $=120+(94 \times 2)$
$=120+188$
Gram molar mass of $\mathrm{Ca}_{3}\left(\mathrm{PO}_{4}\right)_{2}=308 \mathrm{~g}$.

II. Calculation based on number of moles from mass and volume:
Question $1 .$
Calculate the number of moles in $46 \mathrm{~g}$ of sodium.
Solution:
Number of moles $=\frac{\text { Mass of the element }}{\text { Atomic mass of the element }}$
Atomic mass of the element $=\frac{46}{23}=2$ moles of sodium
 

Question 2.
$5.6$ litre of $\mathrm{Oxygen}$ at S.T.P?
Solution:
Given volume of $\mathrm{O}_{2}$ at,
Number of moles $=\frac{\text { S.T.P. }}{\text { Molar volume at S.T.P }}$
Molar volume at S.T.P $=\frac{46}{23}=2$ moles
Number of moles of oxygen $=\frac{5.6}{22.4}=0.25$ mole of oxygen
 

Question $3 .$
Calculate the number of moles of a sample that contains $12.046 \times 10^{23}$ atoms of iron?
Solution:
Number of moles $=\frac{\text { Number of atoms of iron }}{\text { Avogadro's number }}$
$=12.046 \times 10^{23} / 6.023 \times 10^{23}$
$=2$ moles of iron.

III. Calculation of mass from a mole.
Question $1 .$
$0.3$ mole of aluminium (Atomic mass of $\mathrm{Al}=27$ ).
Solution:
Number of moles $=\frac{\text { Mass of } \mathrm{Al}}{\text { Atomic mass of } \mathrm{Al}}$
Mass $=$ No. of moles $\times$ atomic mass
So, mass of $\mathrm{Al}=0.3 \times 27=8.1 \mathrm{~g}$.

Question 2 .
$2.24$ litre of $\mathrm{SO}_{2}$ gas at S.T.P?
Solution:
Molecular mass of $\mathrm{SO}_{2}=32+(16 \times 2)=32+32=64$
Number of moles of $\mathrm{SO}_{2}=\frac{\text { Given volume of } \mathrm{SO}_{2} \text { at S.T.P }}{\text { Molar volume } \mathrm{SO}_{2} \text { at S.T.P }}$ $=\frac{2.24}{22.4}=0.1$ mole
Number of moles $=\frac{\text { Mass }}{\text { Molecular mass }}$
Mass $=$ No. of moles $\times$ molecular mass
Mass $=0.1 \times 64$
Mass of $\mathrm{SO}_{2}=6.4 \mathrm{~g}$.
 

Question $3 .$
$1.51 \times 10^{23}$ molecules of water
Solution:
Molecular mass of $\mathrm{H}_{2} \mathrm{O}=18$
Number of moles $=\frac{\text { Number of molecules of water }}{\text { Avogadro's number }}$
$=1.51 \times 10^{23} / 6.023 \times 10^{23}=1 / 4=0.25$ mole
Number of moles $=\frac{\text { Mass }}{\text { Molecular mass }}$
$0.25=$ mass $/ 18$
Mass $=0.25 \times 18$
Mass $=4.5 \mathrm{~g}$.
 

Question $4 .$
$5 \times 10^{23}$ molecules of glucose?
Solution:
Molecular mass of glucose $=180$
Mass of glucose $=\frac{\text { Molecular mass } \times \text { number of particles }}{\text { Avogadro's number }}$
$=\left(180 \times 5 \times 10^{23}\right) / 6.023 \times 10^{23}$
$=149.43 \mathrm{~g}$.

IV. Calculation based on the number of atoms/molecules.
Question $1 .$
Calculate the number of molecules in $11.2$ litre of $\mathrm{CO}_{2}$ at S.T.P
Solution:
Number of moles of $\mathrm{CO}_{2}=\frac{\text { Volume at S.T.P }}{\text { Molar volume }}$
$=\frac{11.2}{22.4}$
$=0.5$ mole.
Number of molecules of $\mathrm{CO}_{2}=$ Number of moles of $\mathrm{CO}_{2} \times$ Avogadro's number
$=0.5 \times 6.023 \times 10^{23}$
$=3.011 \times 10^{23}$ molecules of $\mathrm{CO}_{2}$
 

Question $2 .$
Calculate the number of atoms present in 1 gram of gold (Atomic mass of Au = 198).
Solution:
Number of atoms of $\mathrm{Au}=\frac{\text { Mass of Au } \times \text { Avogadro's number }}{\text { Atomic mass of Au }}$
Atomic mass of $\mathrm{Au}=\frac{1}{198} \times 6.023 \times 10^{23}$
Number of atoms of $\mathrm{Au}=3.042 \times 10^{21} \mathrm{~g}$.

Question 3 .
Calculate the number of molecules in $54 \mathrm{gm}$ of $\mathrm{H}_{2} \mathrm{O}$
Solution:
Number of molecules $=\frac{(\text { Avogadro number } \times \text { Given mass })}{\text { Gram molecular mass }}$
Number of molecules of water $=6.023 \times 10^{23} \times \frac{54}{18}$ $=18.069 \times 10^{23}$ molecules.
 

Question $4 .$
Calculate the number of atoms of oxygen and carbon in 5 moles of $\mathrm{CO}_{2}$.
Solution:
- 1 mole of $\mathrm{CO}_{2}$ contains 2 moles of oxygen.
- 5 moles of $\mathrm{CO}_{2}$ contains 10 moles of oxygen
Number of atoms of oxygen $=$ number of moles of oxyen $\times$ Avogadro's number $=10 \times 6.023 \times 10^{23}=6.023 \times 10^{24}$ atoms of Oxygen.
- 1 mole of $\mathrm{CO}_{2}$ contains 1 mole of carbon
- 5 moles of $\mathrm{CO}_{2}$ contains 5 moles of carbon
No. of atoms of carbon $=$ No.of moles of carbon $\times$ Avogadro's number $=5 \times 6.023 \times 10^{23}=3.011 \times 10^{24}$ atoms of Carbon.
 

V. Calculation based on molar volume
Calculate the volume occupied by:
Question $1 .$
$2.5$ mole of $\mathrm{CO}_{2}$ at S.T.P.
Solution:
Number of moles of $\mathrm{CO}_{2}=\frac{\text { Given volume at S.T.P }}{\text { Molar volume at S.T.P }}$
$2.5$ mole of $\mathrm{CO}_{2}=\frac{\text { Volume of } \mathrm{CO}_{2} \text { at S.TP }}{22.4}$
Volume of $\mathrm{CO}_{2}$ at S.T.P $=22.4 \times 2.5=56$ litres.

Question 2 .
$3.011 \times 10^{23}$ of ammonia gas molecules?
Solution:
Number of moles $=\frac{\text { Number of molecules }}{\text { Avogadro's number }}$
$=3.011 \times 10^{23} / 6.023 \times 10^{23}$
$=2$ moles
Volume occupied by $\mathrm{NH}_{3}=$ number of moles $\times$ molar volume $=2 \times 22.4=44.8$ litres at S.T.P.
 

Question 3 .
$14 \mathrm{~g}$ nitrogen gas?
Solution:
Number of moles $=\frac{14}{28}=0.5$ mole
Volume occupied by $\mathrm{N}_{2}$ at S.T.P = No. of moles $\times$ molar volume
$=0.5 \times 22.4$
$=11.2$ litres.

VI. Calculation based on \% composition.
Question $1 .$
Calculate $\%$ of $\mathrm{S}$ in $\mathrm{H}_{2} \mathrm{SO}_{4}$
Solution:
Molar mass of $\mathrm{H}_{2} \mathrm{SO}_{4}=(1 \times 2)+(32 \times 1)+(16 \times 4)$ $=2+32+64$
$=98 \mathrm{~g}$.
$\%$ of $\mathrm{S}$ in $\mathrm{H}_{2} \mathrm{SO}_{4}=\frac{\text { Mass of sulphur }}{\text { Molar mass of } \mathrm{H}_{2} \mathrm{SO}_{4}} \times 100$
$\%$ of $\mathrm{S}$ in $\mathrm{H}_{2} \mathrm{SO}_{4}=\frac{32}{98} \times 100$
$=32.65 \%$
 

I. Choose the best answer.
Question $1 .$
Which of the following has the smallest mass?
(a) $6.023 \times 10^{23}$ atoms of He
(b) 1 atom of $\mathrm{He}$
(c) $2 \mathrm{~g}$ of $\mathrm{He}$
(d) 1 mole atoms of $\mathrm{He}$.
Answer:
(b) 1 atom of $\mathrm{He}$
Hint:
(a) $6.023 \times 10^{23}$ atoms of $\mathrm{He}=1$ mole
Mass of 1 mole of $\mathrm{He}=4 \mathrm{~g}$ (or) $0.004 \mathrm{~kg}$.
(b) Mass of 1 atom of $\mathrm{He}=$ ?
Mass of $6.023 \times 10^{23}$ atoms of $\mathrm{He}=0.004 \mathrm{~kg}$.

Mass of 1 atom of $\mathrm{He}=\frac{0.004}{6.023 \times 10^{23}}=6.6423 \times 10^{-27} \mathrm{~kg}$
(c) $2 \mathrm{~g}$ of $\mathrm{He}=$ Mass $=0.002 \mathrm{~kg}$.
(d) 1 mole atoms of $\mathrm{He}=4 \mathrm{~g}=0.004 \mathrm{~kg}$.
So (b) is the smallest mass as $6.6423 \times 10^{-27} \mathrm{~kg}$.
 

Question 2.
Which of the following is a triatomic molecule?
(a) Glucose
(b) Helium
(c) Carbon dioxide
(d) Hydrogen
Answer:
(c) Carbon dioxide
 

Question $3 .$
The volume occupied by $4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$ at S.T.P
(a) $22.4$ litre
(b) $2.24$ litre
(c) $0.24$ litre
(d) $0.1$ litre.
Answer:
(b) $2.24$ litre
Hint:
Molar volume of $\mathrm{CO}_{2}=22.4$ litre.
The volume occupied by 1 mole.
i.e. $44 \mathrm{~g}$ (molar mass) of $\mathrm{CO}_{2}$.
$44 \mathrm{~g}$ of $\mathrm{CO}_{2}$ occupied $22.4$ litre of volume.
$4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$ will occupy $\frac{22,4}{44} \times 4.4=\frac{22,4}{10}=2.24$ litre.
So, answer (b) is correct.

Question $4 .$
Mass of 1 mole of Nitrogen atom is:
(a) $28 \mathrm{amu}$
(b) $14 \mathrm{amu}$
(c) $28 \mathrm{~g}$
(d) $14 \mathrm{~g}$
Answer:
(c) $28 \mathrm{~g}$
 

Question $5 .$
Which of the following represents $1 \mathrm{amu}$ ?
(a) Mass of a $\mathrm{C}-12$ atom
(b) Mass of a hydrogen atom
(c) $\frac{1}{2}$ th of the mass of a $C-12$ atom
(d) Mass of $\mathrm{O}-16$ atom
Answer:
(c) $1 / 12$ th of the mass of a $\mathrm{C}-12$ atom
Hint: By definition 1 amu is defined as precisely $1 / 12$ th the mass of an atom of carbon $-12$. So, answer (c) is correct.

Question $6 .$
Which of the following statement is incorrect?
(a) One gram of $\mathrm{C}-12$ contains Avogadro's number of atoms.
(b) One mole of oxygen gas contains Avogadro's number of molecules.
(c) One mole of hydrogen gas contains Avogadro's number of atoms.
(d) One mole of electrons stands for $6.023 \times 10^{23}$ electrons.
Answer:
(a) One gram of $\mathrm{C}-12$ contains Avogadro's number of atoms.
 

Question $7 .$
The volume occupied by 1 mole of a diatomic gas at S.T.P is
(a) $11.2$ litre
(b) $5.6$ litre
(c) $22.4$ litre
(d) $44.8$ litre.
Answer:
(c) $22.4$ litre
Hint: By definition 1 mole of any gas at S.T.P occupies molar volume i.e. $22.4$ litres.
So (c) is the correct answer.

Question 8 .
In the nucleus of ${ }_{20} \mathrm{Ca}^{40}$, there are
(a) 20 protons and 40 neutrons
(b) 20 protons and 20 neutrons
(c) 20 protons and 40 electrons
(d) 40 protons and 20 electrons.
Answer:
(b) 20 protons and 20 neutrons
Hint:
${ }_{20} \mathrm{Ca}^{40}$
$20=$ Atomic number $=$ Number of protons (or) Number of electrons
$40=$ Mass number $=$ Number of protons $+$ Number of neutrons
${ }_{20} \mathrm{Ca}^{40}$ contains 20 protons, 20 electrons and 20 neutrons.
So the answer (b) is correct.
 

Question $9 .$
The gram molecular mass of oxygen molecule is:
(a) $16 \mathrm{~g}$
(b) $18 \mathrm{~g}$

(c) $32 \mathrm{~g}$
(d) $17 \mathrm{~g}$
Answer:
(b) $18 \mathrm{~g}$
 

Question $10 .$
1 mole of any substance contains _______ molecules.
(a) $6.023 \times 10^{23}$
(b) $6.023 \times 10^{-23}$
(c) $3.0115 \times 10^{23}$
(d) $12.046 \times 10^{23}$.
Answer:
(a) $6.023 \times 10^{23}$
Hint:
Avogadro's law states that 1 mole of any substance contains $6.023 \times 10^{23}$ molecules.
So the answer (a) is correct.
 

II. Fill in the blanks
Question $1 .$
Atoms of different elements having_______ mass number, but_______ atomic numbers are called isobars.
Answer:
Same, different.
 

Question $2 .$
Atoms of different elements having the same number of_______ are called isotones.
Answer:
Neutrons.
 

Question $3 .$
Atoms of one element can be transmuted into atoms of other elements by_______
Answer:
Artificial transmutation.

Question $4 .$
The sum of the numbers of protons and neutrons of an atom is called its_______
Answer:
Mass number.
 

Question $5 .$
Relative atomic mass is otherwise known as_______
Answer:
Standard atomic weight.
 

Question $6 .$
The average atomic mass of hydrogen is_______ amu.
Answer:
$1.008 \mathrm{amu} .$
 

Question $7 .$
If a molecule is made of similar kind of atoms, then it is called_______ atomic molecule.
Answer:
Homo.

Question 8 .
The number of atoms present in a molecule is called its
Answer:
Atomicity.
 

Question $9 .$
One mole of any gas occupies $\mathrm{ml}$ at S.T.P.
Answer:
$22400 .$
 

Question $10 .$
Atomicity of phosphorous is
Answer:
4 .
 

III. Match the following.
Question $1 .$

Answer:
$$
a-i i, b-i i i, c-v, d-i, e-i v
$$
 

IV. True or False: (If false give the correct statement)
Question $1 .$
Two elements sometimes can form more than one compound.
Answer:
True.
 

Question $2 .$
Noble gases are Diatomic
Answer:
False.
Correct Statement: Noble gases are monoatomic
 

Question $3 .$
The gram atomic mass of an element has no unit?
Answer:
False.
Correct Statement: The gram atomic mass of an element is expressed in the unit grams.

Question $4 .$
1 mole of Gold and Silver contain the same number of atoms?
Answer:
True
 

Question $5 .$
The molar mass of $\mathrm{CO}_{2}$ is $42 \mathrm{~g}$ ?
Answer:
False.
Correct Statement: The molar mass of $\mathrm{CO}_{2}$ is $(12+32)=44 \mathrm{~g}$.
 

V. Assertion and Reason:
Answer the following questions using the data given below:
(i) $\mathrm{A}$ and $\mathrm{R}$ are correct, $R$ explains the $\mathrm{A}$.
(ii) $A$ is correct, $R$ is wrong.
(iii) $A$ is wrong, $R$ is correct.
(iv) $\mathrm{A}$ and $\mathrm{R}$ are correct, $R$ doesn't explain $\mathrm{A}$.
 

Question $1 .$
Assertion: Atomic mass of aluminium is 27
Reason: An atom of aluminium is 27 times heavier than $1 / 12$ th of the mass of the $C-12$ atoms.
Answer:
(i) $\mathrm{A}$ and $\mathrm{R}$ are correct, $\mathrm{R}$ explains the $\mathrm{A}$.
 

Question $2 .$
Assertion: The Relative Molecular Mass of Chlorine is $35.5$ a.m.u.
Reason: The natural abundance of Chlorine isotopes are not equal.
Answer:
(i) $\mathrm{A}$ and $\mathrm{R}$ are correct, $\mathrm{R}$ explains the $\mathrm{A}$.
 

VI. Short Answer Questions

Question $1 .$
Define Relative atomic mass.
Answer:
Relative atomic mass of an element is the ratio between the average mass of its isotopes to $1 / 12$ th part of
the mass of a carbon $-12$ atom. It is denoted as $A_{r}$.
Relative atomic mass $=\frac{\text { Average mass of the isotopes of the element }}{1 / 12^{\text {th }} \text { of the mass of one Carbon- } 12 \text { atom }} .$
 

Question $2 .$
Write the different types of isotopes of oxygen and its percentage abundance.
Answer:
Isotopes of oxygen:

The atomic mass of oxygen $=(15.9949 \times 0.99757)+(16.9991 \times 0.00038)+(17.9992 \times 0.00205)=15.999$ amu.
 

Question $3 .$
Define Atomicity.
Answer:
The number of atoms present in the molecule is called atomicity.
 

Question $4 .$
Give any two examples for heteroatomic molecules.
Answer:
Heterodiatomic molecules.
e.g.s
- $\mathrm{HCl}$
- $\mathrm{NaCl}$.
 

Question $5 .$
What is Molar volume of a gas?
Answer:
The volume occupied by one mole of any gas at STP is called molar volume. Its value is equal to $22.4$ litre or $22400 \mathrm{ml}$ or $22400 \mathrm{~cm}^{3}$ or $2.24 \times 10^{-2} \mathrm{~m}^{3}$.
 

Question $6 .$
Find the percentage of nitrogen in ammonia.
Answer:
Ammonia $-\mathrm{NH}_{3}=$ Molar mass $=14+3=17$
Mass $\%$ of Nitrogen $=\frac{14}{17} \times 100=82.35 \%$.
 

VII. Long Answer Questions.

Question $1 .$
Calculate the number of water molecule present in one drop of water which weighs $0.18 \mathrm{~g}$.
Answer:
One mole of water weighs $18 \mathrm{~g}$.
18 g of water contains $6.023 \times 10^{23}$ water molecules.
$\therefore 0.18 \mathrm{~g}$ of water contains
$$
\begin{aligned}
&=\frac{6.023 \times 10^{23}}{18} \times 0.18 \\
&=6.023 \times 10^{21} \text { water molecules }
\end{aligned}
$$
 

Question $2 .$
$\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3}$ (The atomic mass of nitrogen is 14 , and that of hydrogen is 1 )
- 1 mole of nitrogen $(\quad g)+$
- 3 moles of hydrogen $(\quad g) \bar{\rightarrow}$.
- 2 moles of ammonia ( g).
Answer:
$$
\mathrm{N}_{2}+3 \mathrm{H}_{2} \rightarrow 2 \mathrm{NH}_{3}
$$
- 1 mole of $\mathrm{N}_{2}=28 \mathrm{~g}$
- 3 moles of $\mathrm{H}_{2}=6 \mathrm{~g}$
- 2 moles of $\mathrm{NH}_{3}=34 \mathrm{~g}$
$\Rightarrow 1$ mole of nitrogen ( $28 \mathrm{~g})+3$ moles of hydrogen $(6 \mathrm{~g}) \rightarrow 2$ moles of Ammonia ( $34 \mathrm{~g})$.

Question $3 .$
Calculate the number of moles in:
(i) $27 \mathrm{~g}$ of $\mathrm{Al}$
(ii) $1.51 \times 10^{23}$ molecules of $\mathrm{NH}_{4} \mathrm{Cl}$
Answer:
(i) $27 \mathrm{~g}$ of $\mathrm{Al}$
Number of moles $=\frac{\text { Mass }}{\text { Atomic mass }}$
Number of moles in $27 \mathrm{~g}$ of $\mathrm{Al}=\frac{27}{27}=1$ mole.
(ii) $1.51 \times 10^{23}$ molecules of $\mathrm{NH}_{4} \mathrm{Cl}$
Number of moles $=\frac{\text { Number of molecules }}{\text { Avogadro's number }}=\frac{1.51 \times 10^{23}}{6.023 \times 10^{23}}=0.25$ moles. .
 

Question $4 .$
Give the salient features of "Modern atomic theory'.
Answer:
(i) An atom is no longer indivisible.
(ii) Atoms of the same element may have different atomic mass.
Eg: isotopes ${ }_{17} \mathrm{Cl}^{35},{ }_{17} \mathrm{Cl}^{37}$.
(ii) Atoms of different elements may have same atomic masses.

Eg: Isobars $18 \mathrm{Ar}^{40}, 20 \mathrm{Ca}^{40}$.
(iv) Atoms of one element can be transmuted into atoms of other elements. An atom is no longer indestructible.
(v) Atoms may not always combine in a simple whole number ratio.
Eg: Glucose $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6} \mathrm{C}: \mathrm{H}: \mathrm{O}=6: 12: 6$ or $1: 2: 1 .$
(vi) Atom is the smallest particle that takes part in a chemical reaction.
(vii) The mass of an atom can be converted into energy ( $\left.E=m c^{2}\right)$.
 

Question $5 .$
Derive the relationship between Relative molecular mass and Vapour density.
Answer:
(i) The Relative Molecular Mass of a gas or vapour is the ratio between the mass of one molecule of the gas or vapour to mass of one atom of Hydrogen.
(ii) Vapour density is the ratio of the mass of a certain volume of a gas or vapour, to the mass of an equal volume of hydrogen, measured under the same conditions of temperature and pressure.
Vapour Density (V.D.) $=\frac{\text { Mass of a given volume of gas or vapour at S.T.P }}{\text { Mass of same volume of hydrogen }}$.
(iii) According to Avogadro's law, equal volumes of all gases contain equal number of molecules. Thus, let the number of molecules in one volume $=n$, then
(iv) V.D at $\mathrm{STP}=\frac{\text { Mass of "n'molecules of a gas or vapour at S.T.P }}{\text { Mass of 'n'molecules of hydrogen }}$
Cancelling ' $n$ ' which is common, you get
$$
\mathrm{V.D}=\frac{\text { Mass of } 1 \text { molecule of a gas or vapour at S.T.P. }}{\text { Mass of } 1 \text { molecules of hydrogen }} .
$$
(v) Since hydrogen is diatomic
$$
\text { V. D. }=\frac{\text { Mass of } 1 \text { molecule of a gas or vapour at S.T.P. }}{\text { Mass of } 2 \text { atoms of hydrogen }} .
$$

(vi) By comparing the definition of relative molecular mass and vapour density we can write as follows. V. D. $=\frac{\text { Mass of } 1 \text { molecule of a gas or vapour at S.T.P. }}{2 \times \text { Mass of } 1 \text { atom of hydrogen }}$
Relative molecular mass (hydrogen scale) $=\frac{\text { Mass of } 1 \text { molecule of a gas or vapour at STP. }}{\text { Mass of } 1 \text { atom of hydrogen }}$.
(vii) By substituting the relative molecular mass value in vapour density definition, we get Vapour density (V.D.) $=$ Relative molecular mass $/ 2$
$\Rightarrow 2 \times$ vapour density $=$ Relative molecular mass of a gas.
 

VIII. HOT Questions
Question $1 .$
Calcium carbonate is decomposed on heating in the following reaction $\mathrm{CaCO}_{3} \rightarrow \mathrm{CaO}+\mathrm{CO}_{2}$
(i) How many moles of Calcium carbonate are involved in this reaction?

Answer:
One mole
(ii) Calculate the gram molecular mass of calcium carbonate involved in this reaction.
Answer:
Gram molecular mass of $\mathrm{CaCO}_{3}$
$$
\begin{aligned}
&=40+12+3(16) \\
&=100 \mathrm{~g}
\end{aligned}
$$
(iii) How many moles of $\mathrm{CO}_{2}$ are there in this equation?
Answer:
One mole.
 

IX. Solve the following problems.
Question $1 .$
How many grams are there in the following?
Answer:
Formula $=$ No. of moles (n) $\times($ Gram molecular mass $)$
(i) 2 moles of hydrogen molecule, $\mathrm{H}_{2}$
Answer:
Mass of 2 moles of $\mathrm{H}_{2}$ molecule
$=2 \times 2=4 \mathrm{~g}$
(ii) 3 moles of chlorine molecule, $\mathrm{Cl}_{2}$
Answer:
Gram molecular mass of 3 moles of $\mathrm{Cl}_{2}$
$=3 \times 71=213 \mathrm{~g}$

(iii) 5 moles of sulphur molecule, $\mathrm{S}_{2}$
Answer:
Gram molecular mass of 5 moles of $\mathrm{S}_{2}$
$$
\begin{aligned}
&=5 \times 8(32) \\
&=5 \times 256=1280 \mathrm{~g}
\end{aligned}
$$
(iv) 4 moles of phosphorous molecule, $\mathrm{P}_{4}$
Answer:
Gram molecular mass of 4 moles of $P_{2}$
$$
\begin{aligned}
&=4 \times 4(31) \\
&=4 \times 124=496 \mathrm{~g}
\end{aligned}
$$
 

Question $2 .$
Calculate the $\%$ of each element in calcium carbonate. (Atomic mass: $\mathrm{C}-12, \mathrm{O}-16, \mathrm{Ca}-40$ )
Solution:
Calcium carbonate: $\mathrm{CaCO}_{3}$

Molar mass of $\mathrm{CaCO}_{3}=40+12+(16 \times 3)=100 \mathrm{~g}$
$\%$ of Calcium $=\frac{40}{100} \times 100=40 \%$
$\%$ of Carbon $=\frac{12}{100} \times 100=12 \%$
$\%$ of Oxygen $=\frac{48}{100} \times 100=48 \%$.
 

Question $3 .$
Calculate the \% of oxygen in $\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$. (Atomic mass: $\mathrm{Al}-12, \mathrm{O}-16, \mathrm{~S}-32$ )
Solution:
Aluminium Sulphate $-\mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}$
Molar mass of Aluminium Sulphate $=(27 \times 2)+(32 \times 3)+(16 \times 12)=54+96+192=342 \mathrm{~g}$ $\%$ of Oxygen $=\frac{192}{342} \times 100=56.14 \%$.
 

Question $4 .$
Calculate the \% relative abundance of $B-10$ and $B-11$, if its average atomic mass is $10.804 \mathrm{amu}$.
Solution:
The average atomic mass of Boron $=10.804 \mathrm{amu}$.
$\%$ relative abundance of $B-10=$ ?
$\%$ relative abundance of $B-11=$ ?
Let the fraction of relative abundance of $B-10=x$
Let the fraction of relative abundance of $B-11=y$
$x+y=1$
$y=1-x$
Relative abundance $=x(10)+(1-x)(11)=10.804 a m u$
$\Rightarrow 10 \mathrm{x}+11-11 \mathrm{x}=10.804 \mathrm{amu}$
$\Rightarrow 11-\mathrm{x}=10.804 \mathrm{amu}$
$\Rightarrow-\mathrm{x}=10.804-11$
$\Rightarrow-\mathrm{x}=-0.196$
$\Rightarrow \mathrm{x}=0.196$
$x=\%$ abundance of $B-10=0.196 \times 100=19.6 \%$
$y=\%$ abundance of $B-11=100-19.6=80.4 \%$
Percentage abundance of $B-10=19.6 \%$
Percentage abundance of $B-11=80.4 \%$.
 

Activities
Question 1.
Complete the following table by filling the appropriate values / terms

Solution:

Question $2 .$
Classify the following molecules based on their atomicity and fill in the table:
Fluorine $\left(\mathrm{F}_{2}\right)$, Carbon dioxide $\left(\mathrm{CO}_{2}\right)$, Phosphorous $\left(\mathrm{P}_{4}\right)$, Sulphur $\left(\mathrm{S}_{3}\right)$, Ammonia $\left(\mathrm{NH}_{3}\right)$, Hydrogen iodide (HI), Sulphuric Acid $\left(\mathrm{H}_{2} \mathrm{SO}_{4}\right)$, Methane $\left(\mathrm{CH}_{4}\right)$, Glucose $\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)$, Carbon monoxide (CO)

Solution:

Question $3 .$
Under same conditions of temperature and pressure if you collect 3 litres of $\mathrm{O}_{2}, 5$ litres of $\mathrm{Cl}_{2}$ and 6 litres of $\mathrm{H}_{2}$
1. Which has the highest number of molecules?
2. Which has the lowest number of molecules?
Solution:
Number of moles of $\mathrm{O}_{2}=\frac{\text { Volume at S.T.P }}{\text { Molar volume }}=\frac{3}{22.4}=0.1339$ moles
Number of molecules $=$ Number of moles $\times$ Avogadro number
$=0.1339 \times 6.023 \times 10^{23}$
$=0.8064 \times 10^{23}$
$=8.064 \times 10^{22} \mathrm{O}_{2}$ molecules.
Number of moles of $\mathrm{Cl}_{2}=\frac{5}{22.4}=0.2232$ moles
Number of molecules $=0.2232 \times 6.023 \times 10^{23}=1.344 \times 10^{23}$ molecules.
Number of moles of $\mathrm{H}_{2}=\frac{6}{22.4}=0.2678$ moles
Number of molecules $=0.2678 \times 6.023 \times 10^{23}=1.6129 \times 10^{23}$ molecules.
1. 6 litres of $\mathrm{H}_{2}$ has the highest number of molecules.
2. 3 litres of $\mathrm{O}_{2}$ has the lowest number of molecules.

I. Choose the best answer.
Question $1 .$
Which of the following pair indicates isotopes?
(a) ${ }_{17} \mathrm{Cl}^{35},{ }_{17} \mathrm{Cl}^{37}$
(b) ${ }_{18} \mathrm{Ar}^{40},{ }_{, 20} \mathrm{Ca}^{40}$
(c) ${ }_{6} \mathrm{C}^{13}, \mathrm{~N}^{14}$
(d) $33 \mathrm{As}^{77}, \mathrm{Se}^{78}$
Answer:
(a) ${ }_{17} \mathrm{Cl}^{35},{ }_{17} \mathrm{Cl}^{37}$
 

Question $2 .$
The mass of a proton is equal to:
(a) $1 \mathrm{amu}$
(b) $\frac{1}{12^{\text {th }}}$ of the mass of a $\mathrm{C}-12$ atom
(c) zero
(d) both (a) and (b)
Answer:
(d) both (a) and (b)

Question $3 .$
The sum of the number of protons and neutrons of an atom is called its
(a) nucleus
(b) atomic number
(c) mass number
(d) relative atomic mass.
Answer:
(c) mass number
 

Question $4 .$
Total number of electrons present in $1.7 \mathrm{~g}$ of $\mathrm{NH}_{3}$ is:
(a) $6.023 \times 10^{23}$
(b) $6.023 \times 10^{24}$
(c) $6.023 \times 10^{22}$
(d) $6.023 \times 10^{25}$
Answer:
(a) $6.023 \times 10^{23}$
 

Question $5 .$
An isotope of hydrogen without neutrons is
(a) Deuterium $1 \mathrm{H}^{2}$
(b) Protium ${ }_{1} \mathrm{H}^{1}$
(c) Tritium $1 \mathrm{~T}^{3}$
(d) Heavy hydrogen ${ }_{1} D^{2}$.
Answer:
(b) Protium ${ }_{1} \mathrm{H}^{1}$
 

Question $6 .$
The isotope tritium contains 1 proton and neutron in the nucleus.
(a) 1
(b) 2
(c) 3
(d) none
Answer:
(b) 2

Question 7.
Which one of the following element is used as the standard for measuring the relative atomic mass of an element in now a days?
(a) $\mathrm{H}^{2}$
(b) $6^{\mathrm{O}^{13}}$
(c) $\mathrm{C}-12$
(d) $\mathrm{C}-14$.
Answer:
(c) $\mathrm{C}-12$
 

Question 8.
The atom with no neutrons in the nucleus is:
(a) $\mathrm{He}$
(b) Deuterium
(c) Tritium
(d) Protium
Answer:
(d) Protium
 

Question $9 .$
The average atomic mass of carbon is
(a) $12 \mathrm{amu}$
(b) $12.84 \mathrm{amu}$
(c) $24.011 \mathrm{amu}$
(d) $12.011 \mathrm{amu} .$
Answer:
(d) $12.011 \mathrm{amu}$.

Question $10 .$
Which one of the following is the most abundant element in both the Earth's crust and in the human body?
(a) Carbon
(b) Silicon
(c) Oxygen
(d) Hydrogen.
Answer:
(c) Oxygen
 

Question $11 .$
Gram molecular mass of $\mathrm{H}_{2} \mathrm{SO}_{4}$ is:
(a) $49 \mathrm{~g}$
(b) $54 \mathrm{~g}$
(c) $98 \mathrm{~g}$
(d) $100 \mathrm{~g}$
Answer:

(c) $98 \mathrm{~g}$
 

Question $12 .$
Boron $-10$ and Boron $-11$ are called
(a) isotopes
(b) isobars
(c) isotones
(d) isomers.
Answer:
(c) isotopes
 

Question $13 .$
Which of the following are found in the elementary state in nature?

(a) Hydrogen chloride
(b) Carbon dioxide
(c) Noble gases
(d) Oxygen.
Answer:
(c) Noble gases

Question $14 .$
Ammonia gas is formed by the following reaction $\mathrm{N}_{2}(\mathrm{~g})+3 \mathrm{H}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{NH}_{3}(\mathrm{~g})$

The volume of $\mathrm{H}_{2}$ required to form $6 \mathrm{dm}^{3}$ of $\mathrm{NH}_{3}$ is:
(a) $9 \mathrm{dm}^{3}$
(b) $10 \mathrm{dm}^{2}$
(c) $4 \mathrm{dm}^{3}$
(d) $2 \mathrm{dm}^{9}$
Answer:
(a) $9 \mathrm{dm}^{3}$
 

Question $15 .$
Which one of the following is a hetero diatomic molecule?
(a) $\mathrm{O}_{2}$
(6) $\mathrm{N}_{2}$
(c) $\mathrm{HI}$
(d) $\mathrm{CH}_{4}$.
Answer:
(c) HI
 

Question $16 .$
Which one of the following is a hetero triatomic molecule?
(a) $\mathrm{H}_{2} \mathrm{O}$
(b) $\mathrm{BCl}_{3}$
(c) $\mathrm{CH}_{4}$
(d) $\mathrm{PCl}_{5}$.
Answer:
(a) $\mathrm{H}_{2} \mathrm{O}$

Question $17 .$
$1 \mathrm{gm}$ atom of nitrogen represents:
(a) $6.023 \times 10^{2} \mathrm{~N}_{2}$ molecules
(b) $22.4$ litre of $\mathrm{N}_{2}$ at STP
(c) $11.2 \mathrm{~L}$ of $\mathrm{N}_{2}$ at STP
(d) $28 \mathrm{~g}$ of nitrogen
Answer:
(c) $11.2 \mathrm{~L}$ of $\mathrm{N}_{2}$ at STP
 

Question $18 .$
Find out the hetero diatomic molecule?
(a) Hydrogen
(b) Hydrogen chloride
(c) Methane
(d) Ammonia.
Answer:
(b) Hydrogen chloride
 

Question $19 .$
Which one of the following is an example of a polyatomic molecule?
(a) Sulphur
(b) Gold
(c) Sodium
(d) Helium.
Answer:
(a) Sulphur

Question $20 .$
The gram molar mass of $\mathrm{CO}_{2}$ is:
(a) $44 \mathrm{~g}$
(b) $100 \mathrm{~g}$
(c) $4.4 \mathrm{~g}$
(d) $22 \mathrm{~g}$
Answer:
(a) $44 \mathrm{~g}$
 

Question $21 .$
Which one of the following is an example of a polyatomic molecule?
(a) Fluorine
(b) Glucose
(c) Oxygen
(d) Sodium.
Answer:
(b) Glucose $\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)$
 

Question 22 .
The gram molecular mass of water is
(a) 18 amu
(b) $18 \mathrm{~g}$
(c) $18 \mathrm{u}$
(d) $18 .$
Answer:
(b) $18 \mathrm{~g}$
 

Question 23.
The value of Avogadro's number is

(a) $6.023 \times 10^{-23}$
(b) $6.023 \times 10^{23}$
(c) $22.4$
(d) 22400 .
Answer:
(b) $6.023 \times 10^{23}$
 

Question $24 .$
The value of molar volume is
(a) $22.4 \mathrm{ml}$
(b) $22.4$ litres
(c) 22400 litres
(d) $2.24$ litres.
Answer:
(b) $22.4$ litres
 

Question $25 .$
Which one of the following represent Avogadro's law?
(a) $\mathrm{V} \propto \frac{1}{n}$
(b) $\mathrm{V} \propto \mathrm{n}$
(c) $\mathrm{V} \propto \frac{1}{n^{7}}$
(d) $\mathrm{V}^{2} \propto \frac{1}{n}$.
Answer:
(b) $\mathrm{V} \propto \mathrm{n}$
 

Question $26 .$
Which of the following has the highest number of molecules?
(a) 1 litre of $\mathrm{N}_{2}$
(b) 2 litres of oxygen
(c) 5 litres of $\mathrm{Cl}_{2}$
(d) 6 litres of Hydrogen.
Answer:
(d) 6 litres of Hydrogen.
 

Question 27.
Which one of the following has the lowest number of molecules?
(a) 1 litre of $\mathrm{N}_{2}$
(b) 2 litres of $\mathrm{H}_{2}$
(c) 3 litres of $\mathrm{O}_{2}$
(d) 4 litres of $\mathrm{Cl}_{2}$
Answer:
(a) 1 litre of $\mathrm{N}_{2}$

Question $28 .$
$2 \times$ Vapour density is equal to
(a) atomic mass
(b) valency
(c) relative molecular mass
(d) atomic number.
Answer:
(c) relative molecular mass
 

Question $29 .$
The value of gram molar mass of $\mathrm{CO}_{2}$ is
(a) $44 \mathrm{amu}$
(b) $44 \mathrm{~g}$
(c) 44
(d) $44 \mathrm{~kg}$.
Answer:
(b) $44 \mathrm{~g}$
Hint: Molar mass $=12+(16 \times 2)=44 \mathrm{~g}$.
 

Question $30 .$
The number of moles of a sample that contain $36 \mathrm{~g}$ of water is
(a) 1 mole
(b) $0.5$ mole
(c) 4 moles
(d) 2 moles.
Answer:
(d) 2 moles
Hint: $18 \mathrm{~g}$ of water $=1$ mole
$36 \mathrm{~g}$ of water $=\frac{1}{18} \times 36=2$ moles
 

Question $31 .$
Which of the following has the largest number of particles?
(a) $8 \mathrm{~g}$ of $\mathrm{CH}_{4}$
(b) $4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$
(c) $34.2$ g of $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$
(d) $2 \mathrm{~g}$ of $\mathrm{H}_{2}$
Answer:
(d) $2 \mathrm{~g}$ of $\mathrm{H}_{2}$.
Hint. $2 \mathrm{~g}=$ Molar mass $=1$ mole $=6.023 \times 10^{23}$ particles.
Others are less.

Question $32 .$
The number of molecules in $16.0 \mathrm{~g}$ of oxygen is
(a) $6.023 \times 10^{23}$
(b) $6.023 \times 10^{-23}$
(c) $3.01 \times 10^{-23}$
(d) $3.011 \times 10^{23}$
Answer:
(d) $3.011 \times 10^{23}$
Hint: 32 g of oxygen contain $6.023 \times 10^{23}$ molecules.
$16 \mathrm{~g}$ of oxygen will contain
$\frac{6.023 \times 10^{23}}{32} \times 16=3.011 \times 10^{23}$
 

Question $33 .$
The percentage of hydrogen in $\mathrm{H}_{2} \mathrm{O}$ is
(a) $8.88$
(b) $11.2$
(c) $20.60$
(d) $80.0$.
Answer:
(b) $11.2$

Hint: 1 mole of $\mathrm{H}_{2} \mathrm{O}$ has $2.016 \mathrm{~g}$ of $\mathrm{H}_{2}$
Percentage of $\mathrm{H}_{2}=\frac{2.016}{18} \times 100=11.2$
 

Question 34 .
Which of the following contains the largest number of molecules?
(a) $0.2$ mole of $\mathrm{H}_{2}$
(b) $8.0 \mathrm{~g}$ of $\mathrm{H}_{2}$
(c) $17 \mathrm{~g}$ of $\mathrm{H}_{2} \mathrm{O}$
(d) $6.0 \mathrm{~g}$ of $\mathrm{CO}_{2}$
Answer:
(b) $8.0 \mathrm{~g}$ of $\mathrm{H}_{2}$
Hint: No. of moles $=\frac{8}{2}=4$ moles.
No. of molecules $=$ mole $\times$ Avogadro number $=4 \times 6.023 \times 10^{23}=2.409 \times 10^{24}$
 

Question $35 .$
One gram of which of the following contains the largest number of oxygen atoms?
(a) $\mathrm{O}$
(b) $\mathrm{O}_{2}$
(c) $\mathrm{O}_{3}$
(d) All contain the same
Answer:
(c) $\mathrm{O}_{3}$
 

Question $36 .$
The percentage by weight of $\mathrm{O}_{2}$ in $\mathrm{CaSO}_{4} \cdot(\mathrm{O}=16, \mathrm{Ca}=40, \mathrm{~S}=32)$ is
(a) $64 \%$
(b) $28.2 \%$

(c) $47.05 \%$
(d) $16.2 \%$
Answer:
(c) $47.05 \%$
Hint: $\%$ by weight of $\mathrm{O}_{2}=\frac{64}{136} \times 100=47.05 \%$
 

Question $37 .$
One mole of a gas occupies a volume of $22.4 \mathrm{~L}$. This is derived from
(a) Berzilliu's hypothesis
(b) Gay - Lussac's law
(c) Avogadro's law
(d) Dalton's law.
Answer:
(c) Avogadro's law
 

Question 38 .
Volume of gas at STP is $1.12 \times 10^{-7} \mathrm{cc}$. Calculate the number of molecules in it.
(a) $3.011 \times 10^{20}$
(b) $3.011 \times 10^{12}$
(c) $3.011 \times 10^{23}$
(d) $3.011 \times 10^{24}$
Answer:
(b) $3.011 \times 10^{12}$
Hint. $2.24 \times 10^{-3}$ c molecules $6.023 \times 10^{23}$ molecules
$1.12 \times 10-7 \mathrm{cc}$ contains $=\frac{6.023 \times 10^{31}}{22400} \times 1.12 \times 10^{-7}$
$$
=3.011 \times 10^{12} \text {. }
$$
 

Question $39 .$
The number of molecules of $\mathrm{CO}_{2}$ present in $44 \mathrm{~g}$ of $\mathrm{CO}_{2}$ is
(a) $6.023 \times 10^{23}$
(b) $3.011 \times 10^{23}$
(c) $12 \times 10^{23}$
(d) $3 \times 10^{10}$
Answer:
(a) $6.023 \times 10^{23}$ (Avogadro number).

Question 40 .
The volume occupied by $4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$ at S.T.P is
(a) $22.4 \mathrm{~L}$
(b) $2.24 \mathrm{~L}$
(c) $0.224 \mathrm{~L}$
(d) $0.1 \mathrm{~L}$.
Answer:
(b) $2.24 \mathrm{~L}$
Hint. 44 g of $\mathrm{CO}_{2}$ at S.T.P occupies $22.4 \mathrm{~L}$
$4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$ at S.T.P will occupy $\frac{224}{44} \times 4.4=2.24 \mathrm{~L}$.
 

Question $41 .$
How many molecules at present in one gram of hydrogen?
(a) $6.023 \times 10^{23}$
(b) $3.011 \times 10^{23}$
(c) $2.5 \times 10^{23}$
(d) $1.5 \times 10^{23}$
Answer:
(b) $3.011 \times 10^{23}$
Hint: $\mathrm{H}_{2}=$ Molar mass $=2 \mathrm{~g}$
$2 \mathrm{~g}$ of $\mathrm{H}_{2}$ contains $6.023 \times 10^{23}$ molecules
$\therefore 1 \mathrm{~g}$ of $\mathrm{H}_{2}$ will contain $=\frac{6.023 \times 10^{\text {교 }}}{2} \times 1$
$=3.011 \times 10^{23}$ molecules.

Question $42 .$
Atoms which have the same number of protons but different number of neutrons are called
(a) isotopes
(b) isomers
(c) allotropes
(d) isotones.
Answer:
(a) isotopes
 

Question 43 .
Number of atoms which a molecule to sulphur contains is
(a) 3
(b) 8
(c) 4
(d) 2 .
Answer:
(b) $8\left(\mathrm{~S}_{8}\right)$
 

Question $44 .$
An example of a triatomic molecule is
(a) Ozone
(b) Nitrogen
(c) Hydrogen
(d) Ammonia.
Answer:
(a) Ozone

Question $45 .$
The atomic mass of sodium is 23 . The number of moles in $46 \mathrm{~g}$ of sodium is
(a) $0.5$
(b) 2
(c) 1
(d) $0.25$.
Answer:
(b) 2
Hint:
No. of moles $=\frac{\text { Mass }}{\text { Atoms mass }}=\frac{46}{23}=2$.
 

Question $46 .$
The number of atoms in a molecule of the elementary substance is called
(a) Atomic number
(b) Avogadro number
(c) Atomic mass
(d) Atomicity.
Answer:
(d) Atomicity.
 

Question $47 .$
Avogadro number represents the number of atoms in
(a) 12 g of $\mathrm{C}-12$
(b) $4.4 \mathrm{~g}$ of $\mathrm{CO}_{2}$
(c) 320 g of Sulphur
(d) $1 \mathrm{~g}$ of $\mathrm{C}-12$
Answer:
(a) 12 g of $\mathrm{C}-12$
 

Question 48 .
The number of moles in 5 grams of Calcium is
(a) $0.5$ mole
(b) $0.125$ mole
(c) $1.25$ mole
(d) $12.5$ mole.
Answer:
(a) $0.125$ mole
Hint:
No. of moles $=\frac{\text { Mass }}{\text { Atomic mass }}$ $=\frac{5}{40}=\frac{1}{8}=0.125$ mole.

Question $49 .$
One mole of $\mathrm{H}_{2} \mathrm{O}$ corresponds to
(a) $22.4$ litre at $1 \mathrm{~atm}$ and $250^{\circ} \mathrm{C}$
(b) $6.023 \times 10^{23}$ atoms of hydrogen and $6.023 \times 10^{23}$ atoms of oxygen
(c) $18 \mathrm{~g}$
(d) $1 \mathrm{~g}$.
Answer:
(c) $18 \mathrm{~g}$
Hint: One mole $=$ Molar mass $=2+16=18 \mathrm{~g}$.
 

Question 50 .
Which one of the following has the maximum number of atoms?
(a) $18 \mathrm{~g}$ of $\mathrm{H}_{2} \mathrm{O}$
(b) $18 \mathrm{~g}$ of $\mathrm{O}_{2}$
(c) $18 \mathrm{~g}$ of $\mathrm{CO}_{2}$
(d) $18 \mathrm{~g}$ of $\mathrm{CH}_{4}$
Answer:
(d) $18 \mathrm{~g}$ of $\mathrm{CH}_{4}$.
Hint:

Number of atoms $=\frac{\text { Mass of substance }}{\text { Molar mass }}$ $\times \mathrm{N}_{\mathrm{A}} \times$ Number of atoms in the molecule
(a) $18 \mathrm{~g}$ of $\mathrm{H}_{2} \mathrm{O}=\frac{18}{18} \times \mathrm{N}_{\mathrm{A}} \times 3=3 \mathrm{~N}_{\mathrm{A}}$, where $\mathrm{N}_{\mathrm{A}}=6.023 \times 10^{23}$
(b) $18 \mathrm{~g}_{2}$ of $\mathrm{O}_{2}=\frac{18}{32} \times \mathrm{N}_{\mathrm{A}} \times 2=1.12 \mathrm{~N}_{\mathrm{A}}$
(c) $18 \mathrm{~g} \mathrm{of} \mathrm{CO}_{2}=\frac{18}{44} \times \mathrm{N}_{\mathrm{A}} \times 3=1.23 \mathrm{~N}_{\mathrm{A}}$
(d) $18 \mathrm{~g}_{\text {of } \mathrm{CH}_{4}}=\frac{18}{16} \times \mathrm{N}_{\mathrm{A}} \times 5=5.60 \mathrm{~N}_{\mathrm{A}}$ $\therefore(d)$ is the correct answer.
 

Question 51.
The atomicity of $\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}$ is
(a) 9
(b) 11
(c) 10
(d) 12 .
Answer:
(b) 11
 

Question $52 .$
All noble gases are molecules.
(a) diatomic

(b) triatomic
(c) mono atomic
(d) poly atomic.
Answer:
(c) mono atomic
 

Question $53 .$
The total number of atoms represented by the compound $\mathrm{CuSO}_{4} \cdot 5 \mathrm{H}_{2} \mathrm{O}$ is
(a) 27
(b) 21
(c) 5
(d) 8 .
Answer:
(b) 21
 

Question $54 .$
Which one of the following represents the mass of $0.5$ moles of water molecules?
(a) $18 \mathrm{~g}$
(b) $1.8 \mathrm{~g}$
(c) $9 \mathrm{~g}$
(d) $4.5 \mathrm{~g}$.
Answer:
(c) $9 \mathrm{~g}$
Mole $=\frac{\text { Mass }}{\text { Molecular mass }}$
Mass $=$ Mole $\times$ Molecular mass $=0.5 \times 18=9 \mathrm{~g}$.

Question $55 .$
The atomic mass of Calcium is 40 . Calculate the number of moles in $16 \mathrm{~g}$ of Calcium.
(a) $0.4$ mole
(b) 4 moles
(c) 640 moles
(d) $\frac{1}{4}$ mole.
Answer:
(a) $0.4$ mole
Hint:
Mole $=\frac{\text { Mass }}{\text { Atomic mass }}=\frac{16}{40}=\frac{8}{20}=\frac{4}{10}=0.4 \mathrm{~mole}$
 

Question $56 .$
If the atomic mass of sodium is $23 \mathrm{amu}$, then the mass of $3.011 \times 10^{23}$ sodium atoms is
(a) $11.5 \mathrm{~kg}$
(b) $23 \mathrm{~g}$
(c) $0.5$ mole
(d) $11.5 \mathrm{~g}$.
Answer:
(d) $11.5 \mathrm{~g}$.
Hint: Mass of $6.023 \times 10^{23}$ sodium atoms $=23 \mathrm{amu}=23 \mathrm{~g}$.

$\therefore$ Mass of $3.011 \times 10^{23}$ sodium atoms $=\frac{23}{6.023 \times 10^{23}} \times 3.011 \times 10^{23}=11.5 \mathrm{~g}$.
 

Question $57 .$
Which of the following will have maximum mass?
(a) $0.1$ mole of $\mathrm{NH}_{2}$
(b) 1022 atoms of carbon
(c) 1022 molecules of $\mathrm{CO}_{2}$
(d) $1 \mathrm{~g}$ of $\mathrm{Fe}$
Answer:
(a) $0.1$ mole of $\mathrm{NH}_{3}$
Hint:
(a) $0.1$ mole of $\mathrm{NH}_{3}$ has $6.023 \times 10^{23}$ atoms.
Mass of 1 mole of $\mathrm{NH}_{3}=17 \mathrm{~g}$
Mass of $0.1$ mole of $\mathrm{NH}_{3}=1.7 \mathrm{~g}$.
(b) Mass of 1022 atoms of carbon
$6.023 \times 1023 \mathrm{c}$ atoms mass $=12 \mathrm{~g}$
1022 atoms of $\mathrm{C}$ has the mass
$=\frac{12}{6.023 \times 10^{23}} \times 1022=2.036 \times 10^{-20} \mathrm{~g}$.
(c) Mass of 1022 molecules of $\mathrm{CO}_{2}$
$\mathrm{CO}_{2}=$ molar mass $=44 \mathrm{~g}$
$6.023 \times 10^{23} \mathrm{CO}_{2}$ molecules has the mass $=44 \mathrm{~g}$
$\therefore 1022 \mathrm{CO}_{2}$ molecules has the mass 44
$=\frac{44}{6.023 \times 10^{23}} \times 1022=7.466 \times 10^{-20} \mathrm{~g}$.
(d) $1 \mathrm{~g}$ of $\mathrm{Fe}$
$\therefore$ (a) $1.7 \mathrm{~g}$ of $\mathrm{NH}_{3}$ has the highest mass.

Question 58 .
Which of the following correctly represents $360 \mathrm{~g}$ of water?
(i) 2 moles of water
(ii) 20 moles of water
(iii) $6.023 \times 10^{23}$ molecules of water
(iv) $1.2044 \times 10^{25}$ molecules of water
(a) (i) only
(b) (i) and (iv)
(c) (ii) and (iii)
(d) (ii) and (iv).
Answer:
(d) (ii) and (iv).
Hint: (i) 2 moles of water
Mass of 1 mole of water $=18 \mathrm{~g}$
Mass of 2 moles of water $=18 \times 2=36 \mathrm{~g}$.

(ii) 20 moles of water
Mass of 1 mole of water $=18 \mathrm{~g}$
Mass of 20 moles of water $=18 \times 20=360 \mathrm{~g}$.
(iii) $6.023 \times 10^{23}$ molecules of water $=1$ mole $=18 \mathrm{~g}$.
(iv) $1.2044 \times 10^{25}$ molecules of water
$6.23 \times 10^{23}$ molecules of water $=1$ mole
$\therefore 1.2044 \times 10^{25}$ molecules
$=\frac{1}{6.023 \times 10^{23}} \times 1.2044 \times 10^{25}$
$=20$ moles.
$\therefore$ Mass of 20 moles $=20 \times 18=360 \mathrm{~g}$.
So (d) is correct.
 

Question $59 .$
Which of the following contains maximum number of molecules?
(a) $1 \mathrm{~g}$ of $\mathrm{CO}_{2}$
(b) $1 \mathrm{~g}$ of $\mathrm{N}_{2}$
(c) $1 \mathrm{~g}$ of $\mathrm{H}_{2}$
(d) $1 \mathrm{~g}$ of $\mathrm{CH}_{4}$
Answer:
(b) $1 \mathrm{~g}$ of $\mathrm{H}_{2}$
Hint:
(a) $1 \mathrm{~g}$ of $\mathrm{CO}_{2}$
No. of moles $=\frac{\text { Mass }}{\text { Atomic mass }}$ (or) $\frac{\text { Mass }}{\text { Molecular mass }}$
No. of moles of $1 \mathrm{~g}$ of $\mathrm{CO}_{2}=\frac{1}{44}$
No. of molecules $=\frac{1}{44} \times 6.023 \times 10^{23}$
$=1.368 \times 10^{22}$ molecules of $\mathrm{CO}_{2}$.

(b) $1 \mathrm{~g}$ of $\mathrm{N}_{2}$
No. of molecules $=\frac{1}{28} \times 6.023 \times 10^{23}$ $=2.151 \times 10^{22}$ molecules of $\mathrm{N}_{2}$.
(c) $1 \mathrm{~g}$ of $\mathrm{H}_{2}$
No. of molecules $=\frac{6.023 \times 10^{23} \times 1}{2}$ $=3.011 \times 10^{23}$ molecules of $\mathrm{H}_{2}$
(d) $1 \mathrm{~g}$ of $\mathrm{CH}_{4}$
No. of molecules $=\frac{6.023 \times 10^{23} \times 1}{16}$ $=3.764 \times 10^{22}$ molecules of $\mathrm{CH}_{4}$ So (c) is the correct answer.

Question 60 .
Which of the following pair is an example of isotopes?
(a) $21 \mathrm{Sc}^{45}$ and ${ }_{23} \mathrm{~V}^{50}$
(b) ${ }_{22} \mathrm{Ti}^{48}$ and ${ }_{22} \mathrm{Ti}^{50}$
(c) ${ }_{22} \mathrm{Ti}^{50}$ and ${ }_{23} \mathrm{~V}^{50}$
(d) ${ }_{21} \mathrm{Sc}^{45}$ and ${ }_{22} \mathrm{Ti}^{50}$
Answer:
(b) $\left(\right.$ b) ${ }_{22} \mathrm{Ti}^{48}$ and ${ }_{22} \mathrm{Ti}^{50}$.
 

II. Fill in the blanks.
Question $1 .$
Amedeo Avogadro put forward a hypothesis based on the relation between the number of _______and the of _______gases.
Answer:
Molecules, volume.
 

Question $2 .$
The molar volume of a gas at STP is _______and the value of Avogadro Number is_______
Answer:
$22.4 .$ litres, $6.023 \times 10^{23}$
 

Question $3 .$
Nitrogen and oxygen are_______molecules whereas Helium and Neon are_______ molecules.
Answer:
Diatomic, monoatomic
 

Question $4 .$
$1 .$ _______are the building blocks of matter.
$2 .$ _______is a triatomic molecule.

Answer:
1. Atoms and molecules
2. Ozone
 

Question $5 .$
$\mathrm{NH}_{3}, \mathrm{H}_{2} \mathrm{O}$ are_______ molecules whereas $\mathrm{N}_{2}, \mathrm{O}_{2}$ are_______ molecules.
Answer:
Heteroatomic, Homoatomic
 

Question 6 .
_______and_______ are polyatomic molecules.
Answer:
Phosphorous $\left(\mathrm{P}_{4}\right)$, Sulphur $\left(\mathrm{S}_{8}\right)$
 

Question $7 .$
1. Atoms of the same element with same atomic number but a different mass number are called_______
2. Atoms of different elements with the same number of neutrons are called_______

Answer:
1. Isotopes
2. Isotones
 

Question 8.
Atomicity of Nitrogen is whereas the atomicity of Helium is_______
Answer:
2,1 .
 

Question 9.
Atoms of the same element with same atomic number but having different mass number are called_______
Answer:
Isotopes.
 

Question $10 .$
Atoms of different elements with the same atomic mass but a different atomic number are called_______
Answer:
Isobars.
 

Question $11 .$
Atoms of different elements having the same number of neutrons but a different atomic number and different mass number are called_______
Answer:
Isotones.
 

Question 12 .
_______is the smallest particle that takes part in the chemical reaction.
Answer:
Atom.
 

Question $13 .$
Anything that has mass and occupies space is called_______
Answer:
Matter.

Question 14.
Protons and neutrons have considerable mass, but_______ don't have considerable mass.
Answer:
Atoms.
 

Question $15 .$
_______is one-twelfth of the mass of $\mathrm{C}-12$ atom, an isotope of carbon which contains_______ protons and _______neutrons.
Answer:
The atomic mass unit, 6,6 .
 

Question $16 .$
_______are the building blocks of matter.
Answer:
Atoms.
 

Question $17 .$
The stable isotope of _______is used as the standard for measuring the relative atomic mass of an element.

Answer:
Carbon C - 12 .
 

Question $18 .$
Modem methods of determination of atomic mass by_______use $\mathrm{C}-12$ as standard.
Answer:
Mass Spectrometry.
 

Question $19 .$
The relative atomic mass of sulphur is_______
Answer:
32 .
 

Question $20 .$
The average atomic mass of carbon is_______
Answer:
$12.011$ amu.
 

Question $21 .$
The average atomic mass of an element becomes fractional due to the presence of_______
Answer:
Isotopes.
 

Question 22.
_______is the most abundant element in both the Earth's crust and the human body.
Answer:
Oxygen.

Question 23 .
Except for_______atoms of most of the elements are found in the combined form with itself or atoms of other elements.
Answer:
Noble gases.
 

Question $24 .$
A molecule is a combination of two or more atoms held together by_______
Answer:
Chemical bonds.
 

Question $25 .$
If the molecule is made of similar kind of atoms, it is called_______
Answer:
Homo atomic molecule.
 

Question $26 .$
The molecule that consists of atoms of different elements is called_______
Answer:
Hetero atomic molecule.
 

Question $27 .$
The number of_______ present in the molecule is called its atomicity.
Answer:
atoms.
 

Question $28 .$
The atomicity of ozone is_______
Answer:
3 .

Question $29 .$
The atomicity of hydrogen chloride is_______
Answer:
$2 .$
 

Question $30 .$
Water is a _______molecule.
Answer:
Hetero triatomic.
 

Question $31 .$
One mole of an element contains_______ atoms and it is equal to its gram atomic mass.
Answer:
$6.023 \times 10^{23}$
 

Question $32 .$
One mole of any gas occupies _______or_______ at S.T.P.
Answer:
$22.4$ litre, $22400 \mathrm{ml}$
 

Question 33 .
The ________ is useful to determine the empirical formula and molecular formula.
Answer:
Percentage composition.
 

Question $34 .$
The percentage composition of elements is useful to determine _______and_______
Answer:
Empirical formula, molecular formula.
 

Question $35 .$
Avogadro's law is in agreement with_______
Answer:
Dalton's atomic theory.

Question $36 .$
_______determines the relation between molecular mass and vapour density.
Answer:
Avogadro's law.
 

Question $37 .$
Relative molecular mass is equal to_______
Answer:
$2 \times$ Vapour density.
 

Question 38.
Atomicity of sulphur is_______
Answer:
8.
 

Question $39 .$
The metals $\mathrm{Cu}, \mathrm{Ag}, \mathrm{Au}$ are _______elements.
Answer:
Monoatomic.
 

Question $40 .$
The atomicity of $\mathrm{H}_{2} \mathrm{SO}_{4}$ is_______
Answer:
$7 .$
 

Question $41 .$
Atomicity of an element is equal to_______
Answer:
$\frac{\text { Molecular mass }}{\text { Atcanac anass }}$
 

III. Match the following.

Question $1 .$

Answer:
$$
i-c, i i-d, 1 i i-a, i v-b \text {. }
$$
 

Question $2 .$

Answer:
$$
i-b, i i-a, i i i-d, i v-c \text {. }
$$
 

Question $3 .$

Answer:
$$
i-d, \ddot{1 i}-a, i 1 i-b, i v-c \text {. }
$$

Question $4 .$

Answer:
$$
i-d, \text { ii }-c, \text { iii }-b, \text { iv }-a \text {. }
$$

Question $5 .$

Answer:
$$
i-c, \text { ii }-d, \text { iii }-b, i v-a \text {. }
$$

Question $6 .$

Answer:
$$
i-c, i i-d, 11 i-a, i v-b \text {. }
$$

Question $7 .$

Answer:
$$
i-c, i i-a, 11 i-d, i v-b \text {. }
$$

Question $8 .$

Answer:
$$
i-c, \text { ii }-d, \text { iii }-b, i v-a \text {. }
$$

Question $9 .$

Answer:
$$
i-b, i 1-c, i 1 i-d, i v-a
$$

Question $10 .$

Answer:
$i-c, \ddot$
 

IV. State whether true or false. If false, give the correct statement.
Question $1 .$
Isotopes are the atoms of the same element may not be similar in all respects.
Answer:
True.
 

Question $2 .$
Isobars are the atoms of the different elements with the same atomic number and different mass numbers.
Answer:
False.
Correct statement: Isobars are the atoms of the different elements with the same mass number but a different atomic number.
 

Question $3 .$
Isotones are the atoms of different elements with the same number of neutrons.
Answer:
True.

Question $4 .$
The number of molecules present in one mole of an element is called atomicity of an element.
Answer:
False.
Correct statement: The number of atoms present in one molecule of an element is called the atomicity of an element.
 

Question $5 .$
Avogadro's hypothesis is used in the deduction of atomicity of elementary gases.
Answer:
True.
 

Question 6.
The volume of a gas at a given temperature and pressure is proportional to the number of particles.
Answer:
True.
 

Question 7.
The value of Gram molar volume at STP is $11.2$ litres.
Answer:
False.
Correct statement: The value of Gram molar volume at STP is $22.4$ litres.
 

Question 8.
The atomicity of nitrogen, oxygen and hydrogen is two.
Answer:
True.
 

Question $9 .$
Atoms and molecules are the building blocks of matter.
Answer:
True.
 

Question $10 .$
The atoms of certain elements such as hydrogen, oxygen and nitrogen have an independent existence.
Answer:
False.
Correct statement: The atoms of certain elements such as hydrogen, oxygen and nitrogen do not have an independent existence.
 

Question 11 .
A molecule is the simplest structural unit of an element or compound which contains one or more atoms.
Answer:
True.

Question $12 .$
Phosphorous and sulphur are monoatomic molecules.
Answer:
False.
Correct statement: Phosphorous and sulphur are polyatomic molecules.
 

Question $13 .$
$\mathrm{H}_{2} \mathrm{O}, \mathrm{NH}_{3}, \mathrm{CH}_{4}$ are examples of homoatomic molecules.
Answer:
False.
Correct statement: $\mathrm{H}_{2} \mathrm{O}, \mathrm{NH}_{3}, \mathrm{CH}_{4}$ are examples of heteroatomic molecules.
 

Question $14 .$
An atom of one element can be transmuted into an atom of other element is known as artificial transmutation.
Answer:
True.
 

Question $15 .$
The molecule is the smallest particle that takes part in a chemical reaction.
Answer:
False.
Correct statement: Atom is the smallest particle that takes part in a chemical reaction.
 

Question 16.
The sum of the number of protons and neutrons of an atom is called Atomic number.
Answer:
False.
Correct statement: The sum of the number of protons and neutrons of an atom is called mass number.

Question $17 .$
The stable isotope of carbon $(\mathrm{C}-12)$ with atomic mass 12 is used as the standard for measuring the relative atomic mass of an element.
Answer:
True.
 

Question $18 .$
The gram atomic mass of oxygen is $16 \mathrm{~g}$.
Answer:
True.
 

Question $19 .$
Silicon is the most abundant element in the Earth's crust.
Answer:
False.
Correct statement: Oxygen is the most abundant element in the Earth's crust.
 

Question $20 .$
Except for noble gases, atoms of most of the elements are found in the combined form.
Answer:
True.
 

Question $21 .$
The number of atoms present in the molecule is called the Avogadro number.
Answer:
False.
Correct statement: The number of atoms present in the molecule is called its Atomicity.

Question 22,
$\mathrm{O}_{2}, \mathrm{~N}_{2}, \mathrm{H}_{2}, \mathrm{Cl}_{2}, \mathrm{Br}_{2}, \mathrm{~F}_{2}, \mathrm{I}_{2}$ are hetero diatomic molecules.
Answer:
False.
Correct statement: $\mathrm{O}_{2}, \mathrm{~N}_{2}, \mathrm{H}_{2}, \mathrm{Cl}_{2}, \mathrm{Br}_{2}, \mathrm{~F}_{2}, \mathrm{I}_{2}$ are homo diatomic molecules.
 

Question $23 .$
Water is an example of Hetero triatomic molecule.
Answer:
True.
 

Question $24 .$
One molecule of an element contains $6.023 \times 10^{23}$ atoms and it is equal to its gram atomic mass.
Answer:
True.
 

Question $25 .$
An equal volume of all gases under similar conditions of temperature and pressure contain a different number of molecules.
Answer:
False.
Correct statement: Equal volume of all gases under similar conditions of temperature and pressure contain the same number of molecules.
 

Question $26 .$
The mathematical representation of Avogadro'slawisV/n=Constant(or)Vccn(or) $\mathrm{V}=$ Constant $\times \mathrm{n}$.
Answer:
True.
 

Question 27.
The molecular formula of gases can be derived using Avogadro's law.
Answer:
True.
 

Question 28.
The number of moles of a sample that contains $12.046 \times 10^{23}$ atoms of iron is 2 .
Answer:
True.

Question $29 .$
The volume occupied by $14 \mathrm{~g}$ of Nitrogen gas is $22.4$ litres.
Answer:
False.
Correct statement: The volume occupied by $14 \mathrm{~g}$ of Nitrogen gas is $11.2$ litres.
 

Question $30 .$
Avogadro's law determines the relation between molecular mass and absolute density.
Answer:
False.
Correct statement: Avogadro's law determines the relation between molecular mass and vapour density.
 

V. Assertion and Reason
Question $1 .$
Assertion (A): $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$ is not a simple ratio.
Reason (R): The ratio of atoms in a molecule may be fixed and integral but may not be simple.
(a) Both $(A)$ and (R) are correct
(b) Both (A) and (R) are wrong
(c) $(A)$ is correct but ( $R$ ) is wrong
(d) $(A)$ is wrong but (R) is correct.
Answer:
(a) Both $(A)$ and (R) are correct
 

Question $2 .$
Assertion (A): ${ }_{6} \mathrm{C}^{13}$ and ${ }_{7} \mathrm{~N}^{4}$ are called Isotones.
Reason (R): Isotones are the atoms of the different elements with different atomic number but the same mass number.
(a) Both (A) and (R) are correct
(b) Both (A) and (R) are wrong
(c) $(A)$ is correct but (R) is wrong
(d) $(A)$ is wrong but (R) is correct.
Answer:
(c) $(\mathrm{A})$ is correct but ( $R)$ is wrong
 

Question $3 .$
Assertion (A): Nitrogen, oxygen and hydrogen are diatomic molecules.
Reason (R): $\mathrm{N}_{2}, \mathrm{O}_{2}, \mathrm{H}_{2}$ contain two atoms in one molecule and so they are a diatomic molecule.
(a) Both (A) and (R) are correct
(b) Both (A) and (R) are wrong
(c) $(A)$ is correct but (R) is wrong
(d) $(A)$ is wrong but (R) is correct.
Answer:
(a) Both (A) and (R) are correct

Question $4 .$
Assertion (A): Atoms and molecules are the building blocks of matter.
Reason (R): Atom is the ultimate particle of an element which may or may not have an independent existence.
(a) Both (A) and (R) are wrong]
(b) $(A)$ is correct but (R) does not explain (A)
(c) Both (A) and (R) are correct
(d) $(A)$ is wrong but (R) is correct.
Answer:
(c) Both $(A)$ and (R) are correct
 

Question $5 .$
Assertion (A): Hydrogen, Oxygen and Ozone are called homoatomic molecules.
Reason $(R)$ : Homoatomic molecules are made up of atoms of the same element.
(a) Both $(A)$ and (R) are wrong
(b) $(A)$ is correct but (R) is wrong
(c) $(A)$ is wrong but (R) is correct
(d) Both (A) and (R) are correct.
Answer:
(d) Both (A) and ( $R)$ are correct.
 

Question 6.
Assertion (A): Water, Ammonia $\left(\mathrm{H}_{2} \mathrm{O}, \mathrm{NH}_{3}\right)$ are heteroatomic molecules.
Reason (R): Most of the elementary gases and compounds consist of atoms of the same element.
(a) Both (A) and (R) are correct
(b) Both (A) and (R) are wrong
(c) $(A)$ is correct but (R) is wrong
(d) $(A)$ is wrong but (R) is correct.
Answer:
(c) $(A)$ is correct but (R) is wrong

Question $7 .$
Assertion (A): $18 \mathrm{~g}$ water contains Avogadro number $\left(6.023 \times 10^{23}\right)$ of particles.
Reason (R): $18 \mathrm{~g}$ of water is the molecular mass (or) 1 mole of water. One mole is defined as the amount of the substance which contains $6.023 \times 10^{23}$ number of particles.
(a) $(A)$ is correct and (R) explains (A)
(b) $(A)$ is correct but (R) is wrong
(c) $(A)$ is wrong but $(R)$ is correct
(d) Both (A) and (R) are wrong
Answer:
(a) $(A)$ is correct and (R) explains (A)
 

Question $8 .$
Assertion (A): Atoms of the same element may not be similar in all respects.
Reason (R): Atoms of the same element have the same atomic number but a different number of neutrons.
(a) Both $(A)$ and $(R)$ are correct
(b) $(A)$ is correct but (R) is wrong
(c) Both (A) and (R) are wrong
(d) (A) is wrong but (R) is correct.
Answer:
(b) $(A)$ is correct but (R)is wrong
 

Question $9 .$
Assertion (A): The atomicity of ozone is three.
Reason $(R)$ : 1 molecule of ozone contains 3 atoms of oxygen.
(a) Both $(A)$ and (R) are correct
(b) Both $(A)$ and( $R$ ) are wrong
(c) $(A)$ is correct but (R) is wrong
(d) $(A)$ is wrong but ( $R$ ) is correct.
Answer:
(a) Both $(A)$ and (R) are correct

Question $10 .$
Assertion $(A): 1 H^{1}, \quad{ }_{1} \mathrm{H}^{2}, 1 \mathrm{H}^{3}$ are the isotopes of hydrogen.
Reason (R): The atoms of the same element with the same mass number but different at numbers are called isotopes.
(a) Both (A) and (R) are correct
(b) Both (A) and (R) are wrong
(c) $(A)$ is correct but (R) is wrong
(d) (A) is wrong but (R) is correct.
Answer:
(c) (A) is correct but (R) is wrong

Assertion (A) \& Reason (R):
(i) $(A)$ and (R) are correct. (R) explain (A)
(ii) (A) is correct (R) is wrong
(iii) $(A)$ is wrong (R) is correct
(iv) (A) and (R) are correct. (R) does not explain (A).
Question $11 .$
Assertion (A): An atom is no longer indivisible.
Reason (R): The subatomic particles protons, electrons and neutrons were discovered.
Answer:
(i) (A) and (R) are correct. (R) explain (A)
 

Question $12 .$
Assertion (A): ${ }_{18} \mathrm{Ar}^{40}$ and ${ }_{20} \mathrm{Ca}^{40}$ are isobars.
Reason (R): They have the same atomic mass but a different atomic number.
Answer:
(i) A) and (R) are correct; (R) explain (A)
 

Question $13 .$
Assertion (A): ${ }_{17} \mathrm{Cl}^{35}$ and ${ }_{17} \mathrm{CC}^{37}$ are isotones.
Reason (R): Atoms of the same element have the same atomic number but a different mass number.
Answer:
(iii) (A) is wrong (R) is correct

Question 14.
Assertion (A): $\mathrm{NH}_{3}, \mathrm{H}_{2} \mathrm{O}, \mathrm{HCl}$ are heteroatomic molecules.
Reason (R): The molecule that consists of atoms of different elements is called heteroatomic molecules.
Answer:
(i) (A) and (R) are correct. (R) explain (A)
 

Question $15 .$
Assertion (A): ${ }_{6} \mathrm{C}^{13}$ and ${ }_{7} \mathrm{~N}^{14}$ are called isotones.
Reason (R): Atoms of different elements having the same number of neutrons, but a different atomic number and different mass number are called isotones.
Answer:
(i) (A) and (R) are correct. (R) explain (A)
 

VI. Short Answer Questions.
Question $1 .$
What are isotopes? Give an example.
Answer:
Atoms of the same element that have same atomic number but different mass number are called isotopes.
e.g., $, 1 \mathrm{H}^{1},{ }_{1} \mathrm{H}^{2},{ }_{1} \mathrm{H}^{3}$.
 

Question $2 .$
State Avogadro Hypothesis.
Answer:
Equal volumes of all gases under similar conditions of temperature and pressure contain equal number of molecules.
 

Question $3 .$
What are isotones? Give an example.
Answer:
Atoms of different elements having the same number of neutrons but a different atomic number and different mass numbers are called isotones.
e.g., ${ }_{6} \mathrm{C}^{13}{ }_{7} \mathrm{~N}^{14}$.
 

Question $4 .$
Define Mole.
Answer:
Mole is defined as the amount of substance that contains as many specified elementary particles as the number of atoms in $12 \mathrm{~g}$ of $\mathrm{C}-12$ isotope.
It is also defined as the amount of substance which contains Avogadro number $\left(6.023 \times 10^{23}\right)$ of particles.

Question $5 .$
Define
1. Atomic number
2. Mass number
Answer:
1. The atomic number of an element is the number of protons or number of neutrons and electrons present in it.
2. The mass number is the sum of the number of protons and neutrons in an atom.
 

Question 6.
How many grams are there in
(i) 5 moles of $\mathrm{H}_{2} \mathrm{O}$
Answer:
5 moles of $\mathrm{H}_{2} \mathrm{O}=5 \times 18=90 \mathrm{~g}$

(ii) 1 mole of Glucose $\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)$
Answer:
1 mole of Glucose $\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)=180 \mathrm{~g}$
 

Question 7.
Define molecule.
Answer:
A molecule is a combination of two or more atoms held together by the strong chemical force of attraction, i.e. Chemical bonds.
 

Question 8.
What is homo atomic molecule? Give two examples.
Answer:
If the molecule is made of similar kind of atoms, then it is called homoatomic molecule. e.g. $\mathrm{H}_{2}, \mathrm{Cl}_{2}$

Question 9.
What is a heteroatomic molecule? Give two examples.
Answer:
The molecule that consists of atoms of different elements is called a heteroatomic molecule. e.g. $\mathrm{HCl}, \mathrm{H}_{2} \mathrm{O}$
 

Question 10 .
Consider the following and classify them on the basis of their atomicity.
$\mathrm{H}_{2}, \mathrm{CCl}_{4}, \mathrm{O}_{3}, \mathrm{BF}_{3}, \mathrm{HCl}, \mathrm{HNO}_{3}, \mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}, \mathrm{NO}, \mathrm{Cl}_{2}, \mathrm{He}, \mathrm{Au}, \mathrm{P}_{4}$
- Monoatomic molecule - $\mathrm{He}, \mathrm{Au}$
- Homo diatomic molecule $-\mathrm{H}_{2}, \mathrm{Cl}_{2}$
- Homo triatomic molecule $-\mathrm{O}_{3}$
- Homo polyatomic molecule $-\mathrm{P}_{4}$
- Hetero diatomic molecule $-\mathrm{HCl}_{4} \mathrm{NO}$
- Hetero polyatomic molecule - $\mathrm{CCl}_{4}, \mathrm{BF}_{3}, \mathrm{HNO}_{3}, \mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$
 

Question $11 .$
Define Relative molecular mass.
Answer:
The Relative molecular mass of a molecule is the ratio between the mass of one molecule of the substance to $1 / 12$ th mass of an atom of Carbon $-12$ isotope.
 

Question $12 .$
Define Mole.
Answer:
The mole is the amount of the substance that contains as many elementary entities as there are atoms in exactly $12 \mathrm{~g}$ of the carbon - 12 isotope.

Question 13 .
Define the Avogadro number.
Answer:
The actual number of atoms in $12 \mathrm{~g}$ of carbon $-12$ is called the Avogadro number.
It is equal to $6.023 \times 10^{23}\left(\mathrm{~N}_{\mathrm{A}}\right)$.
 

Question $14 .$
What is meant by percentage composition? What is its use?
Answer:
The percentage composition of a compound represents the mass of each element present in $100 \mathrm{~g}$ of the compound. It is useful to determine the empirical formula and molecular formula.
 

Question $15 .$
State Avogadro hypothesis (or) Avogadro's Law.
Answer:
The Avogadro's law states that "equal volume of all gases under similar conditions of temperature and pressure contain the equal number of molecules".
[V $\propto \mathrm{n}]$
 

Question $16 .$
What are the applications of Avogadro's Law?
Answer:
- It explains Gay - Lussac's law.
- It helps in the determination of atomicity of gases.
- The molecular formula of gases can be derived using Avogadro's law.
- It determines the relation between molecular mass and vapour density.
- It helps to determine the gram molar volume of all gases, (i.e, $22.4$ litres at S.T.P).
 

Question 17.
How is Average atomic mass calculated?
Answer:
The average atomic mass of an element is calculated by adding the masses of its isotopes, each multiplied by their natural abundance on the Earth.
 

Question 18 .
Define Vapour density.

Answer:
The vapour density is defined as the ratio between the masses of equal volumes of a gas (or vapour) and hydrogen under the same condition.
 

Question $19 .$
Write the relationship between
1. Atomicity and Molecular mass
2. Molecular mass and Vapour density.
Answer:
1. Atomicity $=\frac{\text { Mokecular mass }}{\text { Atomic mass }}$
2. Molecular mass $=2 \times$ Vapour density
 

Question $20 .$
Distinguish between isotopes and isobars.
Answer:

Question $21 .$
What are the types of molecules? Give an example for each type?
Answer:
Molecules are of two types:
- Homoatomic molecule: The molecules which are made up of atoms of the same element are called Homoatomic molecule, e.g., $\mathrm{N}_{2}, \mathrm{O}_{2}, \mathrm{H}_{2}$
- Heteroatomic molecule: The molecules which are made up of atoms of different elements are called Heteroatomic molecule, e.g., $\mathrm{NH}_{3}, \mathrm{H}_{2} \mathrm{O}_{,} \mathrm{CH}_{4}$
 

VII. HOT Questions.
Question $1 .$
Calculate the mass of $\mathrm{CO}_{2}$ which contains the same number of molecules as are contained in $40 \mathrm{~g}$ of $\mathrm{SO}_{2}$.
Answer:
Gram molecular mass of $\mathrm{SO}_{2}=32+2(16)$
$=64 \mathrm{~g}$
No. of moles of $\mathrm{SO}_{2}=\frac{\text { GiluenMass }}{\text { Mo. Mass }}$
$=\frac{40}{64}=0.625$ moles
$\because$ Equal moles contains equal number of molecules.
Mass of $\mathrm{CO}_{2}$ which contains the same number of molecules,
$=0.625 \times$ mol. mass of $\mathrm{CO}_{2}$
$=0.625 \times 44$
$=27.5 \mathrm{~g}$

Question 2.
A flask P contains $0.5$ moles of oxygen gas. Another flask Q contains $0.4$ moles of ozone gas. Which of the two flasks contains greater number of oxygen atoms?
Answer:
1 molecule of oxygen $\left(\mathrm{O}_{2}\right)=2$ atoms of oxygen
1 molecule of ozone $\left(\mathrm{O}_{3}\right)=3$ atoms of oxygen
In flask $P$ :
1 mole of oxygen gas $=6.022 \times 10^{23}$ molecules
$0.5$ mole of oxygen gas $=6.022 \times 10^{23} \times 0.5$ molecules
$=6.022 \times 10^{23} \times 0.5 \times 2$ atoms
$=6.022 \times 10^{23}$ atoms.
In flask $Q$ :
1 mole of ozone gas $=6.022 \times 10^{23}$ molecules
$0.4$ mole of ozone gas $=6.022 \times 10^{23} \times 0.4$ molecules
$=6.022 \times 10^{23} \times 0.4 \times 3$ atoms
$=7.23 \times 10^{22}$ atoms
Flask $Q$ has a greater number of oxygen atoms as compared to the flask P.
 

Question $3 .$
Chlorophyll, the green pigment of plants responsible for photosynthesis contain $2.68 \%$ of $\mathrm{Mg}$ by weight.
Calculate the number of magnesium atoms in $20 \mathrm{~g}$ of chlorophyll.
Answer:
The weight $\%$ of Mg as $2.68$
i.e., $100 \mathrm{~g}$ of chlorophyll contains $2.68 \mathrm{~g}$ of $\mathrm{Mg}$
$\therefore 2 \mathrm{~g}$ of chlorophyll will contain $\mathrm{Mg}$
$\frac{2.68}{100} \times 20$ $=0.5369$
$=0.5369$
1 mole of $\mathrm{Mg}=24 \mathrm{~g}=6.023 \times 10^{23}$ atoms
$\therefore 0.0536 \mathrm{~g}$ of Mg will have $=\frac{6.023 \times 10^{\text {7f }}}{24} \times 0.536$
$=0.1345 \times 10^{23}$ atoms of $\mathrm{Mg}=1.345 \times 10^{22}$
Number of Magnesium atoms present in $20 \mathrm{~g}$ of chlorophyll is $1.345 \times 10^{22}$
 

Question $4 .$
In three moles of ethane $\left(\mathrm{C}_{2} \mathrm{H}_{6}\right)$, calculate the following:
1. Number of moles of carbon atoms

2. Number of moles of hydrogen atoms
3. Number of molecules of ethane
Answer:
1. 1 mole of $\mathrm{C}_{2} \mathrm{H}_{6}$ contains 2 moles of carbon atoms
3 moles of $\mathrm{C}_{2} \mathrm{H}_{6}$ will $\mathrm{C}$ - atoms $=6$ moles
2. 1 mole of $\mathrm{C}_{2} \mathrm{H}_{6}$ contains 6 moles of hydrogen atoms
3 moles of $\mathrm{C}_{2} \mathrm{H}_{6}$ will contain $\mathrm{H}$-atoms $=18$ moles
3. 1 mole of $\mathrm{C}_{2} \mathrm{H}_{6}$ contains Avogadro's number. i.e., $6.023 \times 10^{23}$ molecules.
3 moles of $\mathrm{C}_{2} \mathrm{H}_{6}$ will contain ethane molecules $=3 \times 6.023 \times 10^{23}=18.06 \times 10^{23}$ molecules.
 

Question $5 .$
If ten volumes of dihydrogen gas react with five volumes of dioxygen gas, how many volumes of water vapour could be produced?
Answer:
$\mathrm{H}_{2}$ and $\mathrm{O}_{2}$ react according to the equation
$\mathrm{H}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g}) \rightarrow 2 \mathrm{H}_{2} \mathrm{O}(\mathrm{g})$
Thus, 2 volumes of $\mathrm{H}_{2}$ react with 1 volume of $\mathrm{O}_{2}$ to produce 2 volumes of water vapour.
Hence, 10 volumes of $\mathrm{H}_{2}$ will react completely with 5 volumes of $\mathrm{O}_{2}$ to produce 10 volumes of water vapour.
 

VIII. Long Answer Questions.
Question $1 .$
What are the differences between atoms and molecules?
Answer:

Question $2 .$
Write the applications of Avogadro's Law.
Answer:
(i) It explains Gay-Lussac's law.
(ii) It helps in the determination of atomicity of gases.
(iii) Molecular formula of the gases can be derived.
(iv) It determines the relation between molecular mass and vapour density.
(v) It helps to determine gram molar volume of all gases
 

Question $3 .$
State and explain the applications of Avogadro's law.
(OR)
Give any two applications of Avogadro's law.
(OR)
Write any three applications of Avogadro's law.
Answer:
Avogadro's law: Equal volumes of all gases under the same conditions of temperature and pressure contain an equal number of molecules.
Applications of Avogadro's law:
- It is used to determine the atomicity of gases.
- It is helpful in determining the molecular formula of gaseous compounds.
- It establishes the relationship between the vapour density and molecular mass of a gas.
- It gives the value of the molar volume of gases at STP. Molar volume of a gas at STP=22.4 litres.
- It explains Gaylussac's law effectively.
 

Question $4 .$
Explain the classification of molecules based on atomicity.
Answer:
In accordance with the number of atoms present in the molecules, they are classified as monoatomic, diatomic, triatomic and polyatomic molecules showing that they contain one, two, three or more than 3 atoms respectively.

Question $5 .$
$\mathrm{A}$ compound made up of two elements $\mathrm{A}$ and $\mathrm{B}$ has $\mathrm{A}=70 \%, \mathrm{~B}=30 \%$. Their relative number of moles in the compound are $1.25$ and $1.88$. Calculate.
(a) Atomic masses of the elements $A$ and $B$.
(b) The molecular formula of the compound, if its molecular mass is found to be 160 .
Answer:

(a) Atomic mass of $\mathrm{A}=\frac{70}{1.25}=56$
Atomic mass of $B=\frac{30}{1.88}=16$
(b) The molecular mass of the compound $=160$
The molecular formula of the compound $=\mathrm{Fe}_{2} \mathrm{O}_{3}$
 

IX. Solve the following problems.
Question $1 .$
Calculate the gram molar mass of the following.
(a) $\mathrm{NaOH}$
(b) $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$
(c) $\mathrm{H}_{3} \mathrm{PO}_{4}$
(Atomic mass of $\mathrm{Na}-23, \mathrm{O}-16, \mathrm{H}-1, \mathrm{C}-12, \mathrm{P}-31$ )
Answer:
(a) $\mathrm{NaOH}$ (Sodium hydroxide)
$\mathrm{GMM}=23+16+1$
$=40 \mathrm{~g}$
Gram molar mass of $\mathrm{NaOH}=40 \mathrm{~g}$
(b) $\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11}$ (Sucrose)
$\mathrm{GMM}=12 \times 12+22 \times 1+11(16)$
$=342 \mathrm{~g}$
Gram molar mass of sucrose $=342 \mathrm{~g}$
(c) $\mathrm{H}_{3} \mathrm{PO}_{4}$ (Phosphoric acid)
$\mathrm{GMM}=3(1)+1(31)+4(16)=98 \mathrm{~g}$
Gram molar mass of Phosphoric acid $=98 \mathrm{~g}$.

Question $2 .$
Calculate the percentage composition of oxygen and hydrogen by taking the example of $\mathrm{H}_{2} \mathrm{O}$
Solution:
Mass $\%$ of an element $=\frac{\text { Mass of that element in the compound }}{\text { Molar mass of the compound }} \times 100$
Now, Molar mass of $\mathrm{H}_{2} \mathrm{O}=2(1)+16=18 \mathrm{~g}$
Mass $\%$ of Hydrogen $=\frac{2}{18} \times 100=11.11 \%$
Mass $\%$ of Oxygen $=\frac{16}{18} \times 100=88.89 \%$
 

Question $3 .$
What is the mass of 1 atom of Gold? (At. mass of Au =197)
Answer:
The mass of $6.023 \times 10^{23}$ atoms of Gold $=197 \mathrm{~g}$
$\therefore$ The mass of 1 atom of gold $=\frac{197}{6.023 \times 10^{23}} \times 1$
$=3.27 \times 10^{-22} \mathrm{~g}$

Question 3 .
What is the mass of 1 atom of Gold? (At. mass of $\mathrm{Au}=197$ )
Answer:
The mass of $6.023 \times 10^{23}$ atoms of Gold $=197 \mathrm{~g}$
$\therefore$ The mass of 1 atom of gold $=\frac{197}{6.023 \times 10^{23}} \times 1$
$=3.27 \times 10^{-22} \mathrm{~g}$
 

Question $4 .$
Find the gram molecular mass of the following from the data given:
(i) $\mathrm{H}_{2} \mathrm{O}$
(ii) $\mathrm{CO}_{2}$
(iii) $\mathrm{NaOH}$
(iv) $\mathrm{NO}_{2}$
(v) $\mathrm{H}_{2} \mathrm{SO}_{4}$

Solution:
(i) $\mathrm{H}_{2} \mathrm{O}$
Atomic mass of $2(\mathrm{H})=2 \times 1=2$
Atomic mass of $1(\mathrm{O})=1 \times 16=16$
Molecular mass of $\mathrm{H}_{2} \mathrm{O}=2+16=18$
(ii) $\mathrm{CO}_{2}$
Atomic mass of $1(\mathrm{C})=1 \times 12=12$
Atomic mass of $2(\mathrm{O})=2 \times 16=32$
Molecular mass of $\mathrm{CO}_{2}=12+32=44 \mathrm{~g}$
(iii) $\mathrm{NaOH}$
Atomic mass of $1(\mathrm{Na})=1 \times 23=23$
Atomic mass of $1(0)=1 \times 16=16$
Atomic mass of $1(\mathrm{H}) 1 \times 1=1$
Molecular mass of $\mathrm{NaOH}=23+16+1=40 \mathrm{~g}$
(iv) $\mathrm{NO}_{2}$
Atomic mass of $1(\mathrm{~N})=1 \times 14=14$
Atomic mass of $2(\mathrm{O})=2 \times 16=32$
Molecular mass of $\mathrm{NO}_{2}=14+32=46 \mathrm{~g}$.
(v) $\mathrm{H}_{2} \mathrm{SO}_{24}$
Atomic mass of $2(\mathrm{H})=2 \times 1=2$
Atomic mass of $1(\mathrm{~S})=1 \times 32=32$
Atomic mass of $4(\mathrm{O})=4 \times 16=64$
Molecular mass of $\mathrm{H}_{2} \mathrm{SO}_{4}=64+32+2=98 \mathrm{~g}$.

Question $5 .$
Complete the table given below.

Solution:

Question $6 .$
Fill in the blanks using the given data:
The formula of Calcium oxide is $\mathrm{CaO}$. The atomic mass of Ca is $40, \mathrm{Oxygen}$ is 16 and Carbon is $12 .$
- 1 mole of Ca (.... g) and 1 mole of the Oxygen atom (.....g) combine to form mole of CaO (.... g).
- 1 mole of Ca $(\ldots . . \mathrm{g})$ and 1 mole of $\mathrm{C}(\ldots . \mathrm{g})$ and 3 moles of the Oxygen atom (.... g) combine to form 1 mole of $\mathrm{CaCO}_{3}(\ldots . \mathrm{g})$.
Solution:
- 1 mole of Ca (40 g) and 1 mole of the Oxygen atom (16 g) combine to form 1 mole of CaO ( $56 \mathrm{~g})$.

- 1 mole of Ca $(40 \mathrm{~g})$ and 1 mole of $\mathrm{C}(12 \mathrm{~g})$ and 3 moles of the Oxygen atom ( $48 \mathrm{~g})$ combine to form 1 mole of $\mathrm{CaCO}_{3}(100 \mathrm{~g})$
 

Question $7 .$
Calculate the average atomic mass of naturally occurring magnesium using the following data.
$$
\mathrm{Mg}-24=78.99 \%, \mathrm{Mg}-25=10 \%, \mathrm{Mg}-26=11.01 \%
$$
Answer:
Average atomic mass of Magnesium $=$ atomic mass of Mg $-24 \times \%+$ atomic mass of Mg $-25 \times \%+$ atomic mass of $\mathrm{Mg}-26 \times \%$
$$
\begin{aligned}
&=24 \times \frac{78.99}{100}+25 \times \frac{10}{100}+26 \times \frac{11.01}{100} \\
&=18.9576+2.5+2.8626
\end{aligned}
$$
$==24.3202 \mathrm{amu}$
$\therefore$ Average atomic mass of Magnesium is $24.3202 \mathrm{amu}$
 

Question $8 .$
Analyse the table and fill in the blanks.

Solution:

Question $9 .$
Analyse the table and fill in the blanks.

Solution:

Question $10 .$
When ammonia reacts with hydrogen chloride gas, it produces white fumes of ammonium chloride. The volume occupied by $\mathrm{NH}_{3}$ in glass bulb $\mathrm{A}$ is three times more than the volume occupied by $\mathrm{HCl}$ in glass bulb B at STP.

(i) How many moles of ammonia are present in glass bulb A?
(ii) How many grams of $\mathrm{NH}_{4} \mathrm{Cl}$ will be formed when the stopper is opened? (Atomic mass of $\mathrm{N}=14, \mathrm{H}=$ $1, \mathrm{Cl}=35.5)$
(iii) Which gas will remain after completion of the reaction?
(iv) Write the chemical reaction involved in this process.
Solution:
(i) Capacity of $\mathrm{NH}_{3}$ bulb $=67.2$ litre
$22.4$ litre of $\mathrm{NH}_{3}=1$ mole
$67.2$ litre of $\mathrm{NH}_{3}=\frac{1}{22.4} \times 67.2=3$ moles of $\mathrm{NH}_{3}$
(ii) Atomic mass of $1(\mathrm{~N})=1 \times 14=14 \mathrm{~g}$
Atomic mass of $4(\mathrm{H})=4 \times 1=4 \mathrm{~g}$
Atomic mass of $1(\mathrm{Cl})=1 \times 35.5=35.5 \mathrm{~g}$
Mass of $\mathrm{NH}_{4} \mathrm{Cl}=53.5 \mathrm{~g}$.
(iii) NH3 (Ammonia) gas will remain after the completion of the reaction.
(iv) Chemical equation of the reaction
$\mathrm{NH}_{3}$ (Ammonia) $+\mathrm{HCl}$ (Hydrochloric acid) $\rightarrow \mathrm{NH}_{4} \mathrm{Cl}$ (Ammonium chloride)
 

Question $11 .$
Nitroglycerine is used as an explosive. The equation for the explosive reaction is
$4 \mathrm{C}_{3} \mathrm{H}_{5}\left(\left(\mathrm{NO}_{3}\right)\right)_{3}(\mathrm{l}) \rightarrow 12 \mathrm{CO}_{2}(\mathrm{~g})+10 \mathrm{H}_{2} \mathrm{O}(\mathrm{l})+6 \mathrm{~N}_{2}(\mathrm{~g})+\mathrm{O}_{2}(\mathrm{~g})$
(Atomic mass of $\mathrm{C}=12, \mathrm{H}=1, \mathrm{~N}=14, \mathrm{O}=16$ )
(i) How many moles does the equation show for:

(a) Nitroglycerine
(b) gas molecules produced?
(ii) How many moles of gas molecules are obtained from 1 mole of nitroglycerine?
(iii) What is the mass of 1 mole of nitroglycerine?
Solution:
(i) 4 moles of Nitroglycerine
(ii) 4 moles of Nitroglycerine produce 19 moles of gas molecules
1 mole of Nitroglycerine produces $19 / 4=4.75$ moles
(iii) Mass of 1 mole of Nitroglycerine $\mathrm{C}_{3} \mathrm{H}_{5}\left(\mathrm{NO}_{3}\right)_{3}$
Atomic mass of $C=12$
Atomic mass of $3(\mathrm{C})=3 \times 12=36$
Atomic mass of $5(\mathrm{H})=5 \times 1=5$
Atomic mass of $3(\mathrm{~N})=3 \times 14=42$
Atomic mass of $9(\mathrm{O})=9 \times 16=144$
Mass of 1 mole of Nitroglycerine $=227 \mathrm{~g}$
 

Question $12 .$
Sodium bicarbonate breaks down on heating:
$2 \mathrm{NaHCO}_{3} \rightarrow \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2}$
(Atomic mass of $\mathrm{Na}=23, \mathrm{C}=12, \mathrm{H}=1, \mathrm{O}=16$ )
(i) How many moles of sodium bicarbonate are there in this equation?
(ii) What is the mass of sodium bicarbonate used in this equation?
(iii) How many moles of carbon dioxide are there in this equation?
Solution:
$2 \mathrm{NaHCO}_{3} \stackrel{\Delta}{\longrightarrow} \mathrm{Na}_{2} \mathrm{CO}_{3}+\mathrm{H}_{2} \mathrm{O}+\mathrm{CO}_{2} \uparrow$
(i) 2 moles of $\mathrm{NaHCO}_{3}$ (sodium bicarbonate) are there in the above equation.
(ii) Mass of sodium bicarbonate in this equation is mass of 2 moles of $\mathrm{NaHCO}_{3}$.
Atomic mass of $1(\mathrm{Na})=1 \times 23=23 \mathrm{~g}$
Atomic mass of $1(\mathrm{H})=1 \times 1=1 \mathrm{~g}$
Atomic mass of $1(\mathrm{C})=1 \times 12=12 \mathrm{~g}$
Atomic mass of $3(\mathrm{O})=3 \times 16=48 \mathrm{~g}$
Mass of 1 mole of $\mathrm{NaHCO}_{3}=84 \mathrm{~g}$
Mass of 2 moles of $\mathrm{NaHCO}_{3}=84 \times 2=168 \mathrm{~g}$.

(iii) Number of moles of $\mathrm{CO}_{2}$ in this equation $=1$ mole.
 

Question $13 .$
$40 \mathrm{~g}$ of calcium was extracted from $56 \mathrm{~g}$ of calcium oxide (Atomic mass of $\mathrm{Ca}=40, \mathrm{O}=16$ )
(i) What mass of oxygen is there in $56 \mathrm{~g}$ of calcium oxide?
(ii) How many moles of oxgen atoms are there in this?
(iii) How many moles of calcium atoms are there in $40 \mathrm{~g}$ of calcium?
(iv) What mass of calcium will be obtained from $1000 \mathrm{~g}$ of calcium oxide?
Solution:
(i) Mass of $\mathrm{CaO}=56 \mathrm{~g}$
Mass of $\mathrm{Ca}=40 \mathrm{~g}$
Mass of oxygen $=56-40=16 \mathrm{~g}$
(ii) $\frac{\text { No of moles of oxygen atom }}{\text { Mole }=\text { mass/atomic mass }}=\frac{16}{16}=1$ mole
1 mole ox oxgen atom.

(iii) $\frac{\text { No of moles of calcium }}{\text { Mole = mass/atomic mass }}=\frac{40}{40}=1$ mole
1 mole of calcium atom.
(iv) $56 \mathrm{~g}$ calcium oxide gives $40 \mathrm{~g}$ of calcium
$1000 \mathrm{~g}$ of calcium oxide give $=\frac{40}{56} \times 1000$
$=714.285 \mathrm{~g}$ of calcium
$=714.29 \mathrm{~g}$ of calcium.
 

Question 14.
How many grams are there in the following?
(i) 1 mole of chlorine molecule, $\mathrm{Cl}_{2}$
(ii) 2 moles of sulphur molecules, $\mathrm{S}_{8}$
(iii) 4 moles of ozone molecules, $\mathrm{O}_{3}$
(iv) 2 moles of nitrogen molecules, $\mathrm{N}_{2}$
Solution:
(i) 1 mole of chlorine molecule $\mathrm{Cl}_{2}$
Atomic mass of chlorine $=35.5 \mathrm{~g}$
Mass of 1 mole of chlorine $=$ Atomic mass $\times$ Atomicity $=35.5 \times 2=71 \mathrm{~g}$.
(ii) 2 moles of sulphur molecules $\mathrm{S}_{8}$
Atomic mass of $\mathrm{S}_{8}=8 \times 32=256 \mathrm{~g}$
Mass of 2 moles of $S_{8}=$ Atomic mass $\times$ Number of moles $=256 \times 2=512 \mathrm{~g}$.
(iii) 4 moles of ozone molecule $\mathrm{O}_{3}$
Atomic mass of $\mathrm{O}_{3}=3 \times 16=48 \mathrm{~g}$
Mass of 4 moles of ozone $=48 \times 4=192 \mathrm{~g}$.

(iv) 2 moles of Nitrogen molecule $\mathrm{N}_{2}$
Atomic mass of $\mathrm{N}_{2}=2 \times 14=28 \mathrm{~g}$
Mass of 2 moles of Nitrogen $=28 \times 2=56 \mathrm{~g}$.
 

Question $15 .$
Find how many moles of atoms are there in:
(i) 2 g of nitrogen
(ii) $23 \mathrm{~g}$ of sodium
(iii) $40 \mathrm{~g}$ of calcium
(iv) $1.4 \mathrm{~g}$ of lithium
(v) $32 \mathrm{~g}$ of sulphur.
Solution:
(i) $2 \mathrm{~g}$ of nitrogen
Number of moles $=\frac{\text { Mass }}{\text { atomic mass }}$
Atomic mass of Nitrogen $=14$; Mass of Nitrogen $=2 \mathrm{~g}$
Number of moles $=\frac{2}{14}=0.142$ moles of Nitrogen.

(ii) $23 \mathrm{~g}$ of sodium
Atomic mass of sodium $=23$
Mass of sodium $=23 \mathrm{~g}$
Number of moles $=\frac{\text { Mass }}{\text { atomic mass }}=\frac{23}{23}=1$ mole of sodium.
(iii) $40 \mathrm{~g}$ of calcium
Atomic mass of calcium $=40$
Mass of calcium $=40 \mathrm{~g}$
Number of moles $=\frac{\text { Mass }}{\text { atomic mass }}=\frac{40}{40}=1$ mole of calcium.
(iv) $1.4 \mathrm{~g}$ of lithium
Atomic mass of lithium $=7$
Mass of lithium $=1.4 \mathrm{~g}$
Number of moles $=\frac{\text { Mass }}{\text { atomic mass }}=\frac{1.4}{7}=0.2$ mole of lithium.
(v) $32 \mathrm{~g}$ of sulphur
Atomic mass of sulphur $=32$
Mass of sulphur $=32 \mathrm{~g}$
Number of moles $=\frac{\text { Mass }}{\text { atornic mass }}=\frac{32}{32}=1$ mole of sulphur.
 

Question $16 .$
Find the atomicity of chlorine, if its atomic mass is $35.5$ and its molecular mass is 71 .
Solution:
Atomicity $=\frac{\text { Molecuhy mass }}{\text { Atowa mass }}$
Atomicity of chlorine $=\frac{71}{35.5}=2$.
 

Question $17 .$
Find the atomicity of ozone if its atomic mass is 16 and its molecular mass is 48 .
Solution:
Atomicity $=\frac{\text { Molecuhur mass }}{\text { Atomx mass }}$
Atomicity of chlorine $=\frac{48}{16}=3$.
 

Question $18 .$
How many atoms are present in 5 moles of oxygen?
Solution:

One mole of oxygen contains $6.023 \times 1023$ atoms
5 moles of oxygen contain $=5 \times 6.023 \times 10^{23}$
$$
\begin{aligned}
&=30.115 \times 10^{23} \\
&=3.0115 \times 10^{24} \text { atoms. }
\end{aligned}
$$
 

Question $19 .$
Calculate the number of moles in
1. $81 \mathrm{~g}$ of Aluminium
2. $4.6 \mathrm{~g}$ of sodium
3. $5.1 \mathrm{~g}$ of ammonia
4. $90 \mathrm{~g}$ of water
$5.2 \mathrm{~g}$ of $\mathrm{NaOH}$.
Solution:
No of moles $=\frac{\text { Given mass }}{\text { Atcom mass }}$
1. No. of moles of Aluminium $=\frac{81}{27}=3$ moles of aluminium
2. No. of moles of Sodium $=\frac{4.6}{23}=0.2$ moles of sodium
3. No. of moles of Ammonia $=\frac{5.1}{17}=0.3$ moles of ammonia
4. No. of moles of Water $=\frac{90}{18}=5$ moles of water
5. No. of moles of $\mathrm{NaOH}=\frac{2}{40}=0.05$ moles of $\mathrm{NaOH}$
 

Question $20 .$
Calculate the mass of $0.5$ moles of iron.
Solution:
Mass = Atomic mass $\times$ number of moles
Mass of iron $=55.9 \times 0.5=27.95 \mathrm{~g}$.
 

Question $21 .$
Find the mass of $2.5$ moles of oxygen atoms.
Solution:
Mass = Atomic mass $\times$ number of moles
Mass of oxygen $=16 \times 2.5=40 \mathrm{~g}$.

Question $22 .$
Calculate the number of molecules in $11 \mathrm{~g}$ of $\mathrm{CO}_{2}$.
Solution:
Gram molecular mass of $\mathrm{CO}_{2}=44 \mathrm{~g}$
Number of molecules $=\frac{\text { Avogadro number } \times \text { given mase }}{\text { Cifam molecribur mass }}$
$$
\begin{aligned}
&=\frac{6.023 \times 10^{27} \times 11}{44} \\
&=1.51 \times 10^{23} \mathrm{CO}_{2} \text { molecules. }
\end{aligned}
$$
 

Question $23 .$
Calculate the number of molecules in $360 \mathrm{~g}$ of glucose.
Solution:
Number of molecules $=\frac{\text { Avagadro mumher } \times \text { given mass }}{\text { Gram mohar mass }}$ Gram molar mass of glucose $\left(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}\right)=(6 \times 12)+(12 \times 1)+(6 \times 16)$ $=72+12+96$
$=180 \mathrm{~g}$
Number of molecules $=\frac{6.093 \times 10^{23} \times 360}{180}$
$=6.023 \times 10^{23} \times 2$
$=12.046 \times 10^{23}$ molecules.
(or)
$1.2046 \times 10^{24}$ glucose molecules.

Question $24 .$
Calculate the mass of $18.069 \times 10^{23}$ molecules of $\mathrm{SO}_{2}$ ?
Solution:
Mass of the substance $=\frac{\text { Gram molecular mass } \times \text { Number of particles }}{\text { Avogadro number }}$
Gram molecular mass of $\mathrm{SO}_{2}=32+16(2)=64 \mathrm{~g}$
Mass of $\mathrm{SO}_{2}=\frac{64 \times 18.069 \times 10^{23}}{6.023 \times 10^{23}}$
$$
=64 \times 3=192 \mathrm{~g} \text {. }
$$
 

Question 25 .
Calculate the mass of glucose in $2 \times 10^{24}$ molecules.
Solution:
Gram molecular mass of glucose $=180 \mathrm{~g}$
Mass of glucose $=\frac{180 \times 2 \times 10^{24}}{6.023 \times 10^{23}}=597.7 \mathrm{~g}$
 

Question $26 .$
Calculate the mass of $12.046 \times 10^{23}$ molecules of $\mathrm{CaO}$.
Solution:
Gram molecular mass of $\mathrm{CaO}=40+16=56 \mathrm{~g}$
Mass of $\mathrm{CaO}=\frac{56 \times 12.046 \times 10^{21}}{6.023 \times 10^{23}}$
$56 \times 2=112 \mathrm{~g}$.

Question $27 .$
Calculate the number of moles for a substance containing $3.0115 \times 10^{23}$ molecules in it.

Solution:
Number of moles $=\frac{\text { No. of molecules }}{\text { Avogadro number }}$
$$
\begin{aligned}
&=\frac{12.046 \times 10^{22}}{6.023 \times 10^{23}} \\
&=0.5 \text { mole. }
\end{aligned}
$$
 

Question $28 .$
Calculate the number of moles in $12.046 \times 1022$ atoms of copper.
Solution:
No. of moles of atoms
$$
\begin{aligned}
&=\frac{\text { No. of atoms }}{\text { Avogadro number }} \\
&=\frac{12.046 \times 10^{22}}{6.023 \times 10^{23}}=2 \times 10^{-1} . \\
&=0.2 \text { mole. }
\end{aligned}
$$

Question $29 .$
Calculate the number of moles in $24.092 \times 10^{22}$ molecules of water.
Solution:
Number of moles $=$
$$
\begin{aligned}
&=\frac{\text { No. of molecules }}{\text { Avogadro number }} \\
&=\frac{24.092 \times 10^{22}}{6.023 \times 10^{23}} \\
&=4 \times 1022 \times 10^{-23} \\
&=4 \times 10^{-1} \\
&=0.4 \text { mole. }
\end{aligned}
$$
 

Question 30 .
Which one of the following will have largest number of atoms?
1. $1 \mathrm{~g} \mathrm{Au}(\mathrm{s})$
2. $1 \mathrm{~g} \mathrm{Na}$ (s)
3. $1 \mathrm{~g} \mathrm{Li}(\mathrm{s})$
4. $1 \mathrm{~g}$ of $\mathrm{Cl}_{2}(\mathrm{~g})$
(Atomic masses: $\mathrm{Au}=197, \mathrm{Na}=23, \mathrm{Li}=7, \mathrm{Cl}=35.5 \mathrm{amu}$ )
Solution:
1. $1 \mathrm{~g} \mathrm{Au}=\frac{1}{197} \mathrm{~mol}=\frac{1}{197} \times 6.02 \times 10^{23}$ atoms
2. $1 \mathrm{~g} \mathrm{Na}=\frac{1}{23} \mathrm{~mol}=\frac{1}{23} \times 6.02 \times 10^{23}$ atoms
3. $1 \mathrm{~g} \mathrm{Li}=\frac{1}{7} \mathrm{~mol}=\frac{1}{7} \times 6.02 \times 10^{23}$ atoms
4. $1 \mathrm{~g} \mathrm{Cl}_{2}=\frac{1}{71} \mathrm{~mol}=\frac{1}{71} \times 6.02 \times 10^{23}$ molecules $=\frac{2}{71} \times 6.02 \times 10^{23}$ atoms.
Thus, $1 \mathrm{~g}$ of Li has the largest number of atoms.

Question 31 .
Calculate the number of atoms in each of the following:
(i) 52 moles of $\mathrm{He}$
(ii) $52 \mathrm{u}$ of $\mathrm{He}$
(iii) $52 \mathrm{~g}$ of $\mathrm{He}$
Solution:
(i) $1 \mathrm{~mol}$ of $\mathrm{He}=6.022 \times 10^{23}$ atoms
$52 \mathrm{~mol}$ of $\mathrm{He}=52 \times 6.022 \times 10^{23}$ atoms $=3.131 \times 10^{25}$ atoms.
(ii) 1 atom of $\mathrm{He}=4$ u of $\mathrm{He}$
$4 \mathrm{u}$ of $\mathrm{He}=1$ atom of $\mathrm{He}$
$52 \mathrm{u}$ of $\mathrm{He}=\frac{1}{4} \times 52$ atoms $=13$ atoms.
(iii) 1 mole of $\mathrm{He}=4 \mathrm{~g}=6.022 \times 10^{23}$ atoms
$52 \mathrm{~g}$ of $\mathrm{He}=\frac{6.022 \times 10^{23}}{4} \times 52$ atoms $=7.8286 \times 1024$ atoms.

Question $32 .$
Calculate the number of moles in each of the following.
(i) $392 \mathrm{~g}$ of sulphuric acid
(ii) $44.8$ litres of sulphur dioxide at N.T.P.
(iii) $6.022 \times 1022$ molecules of oxygen
(iv) $8 \mathrm{~g}$ of calcium
Solution:
(i) $392 \mathrm{~g}$ of sulphuric acid
Molar mass of $\mathrm{H}_{2} \mathrm{SO}_{4}=2 \times 1+32+4 \times 16=98 \mathrm{~g}$
$98 \mathrm{~g}$ of sulphuric acid $=1 \mathrm{~mol}$
$392 \mathrm{~g}$ of sulphuric acid $=1 \mathrm{~mol} \times \frac{392 g}{(98 g)}=4 \mathrm{~mol} .$
(ii) $44.8$ litres of sulphur dioxide at N.T.P.
$22.4$ litres of sulphur dioxide at N.T.P. $=1 \mathrm{~mol}$
$44.8$ litres of sulphur dioxide at N.T.P. $=\frac{1 \mathrm{~mol}}{(22.4 \mathrm{~L})} \times(44.8 \mathrm{~L})=2.0 \mathrm{~mol} .$
(iii) $6.022 \times 10^{22}$ molecules of oxygen
$6.022 \times 10^{22}$ molecules of oxygen $=1 \mathrm{~mol}$
$6.022 \times 10^{22}$ molecules of oxygen $=1 \mathrm{~mol} \times \frac{6.022 \times 10^{22}}{6.022 \times 10^{23}}=0.1 \mathrm{~mol}$.
(iv) $8 \mathrm{~g}$ of calcium
Gram atomic mass of $\mathrm{Ca}=40 \mathrm{~g}$
$40 \mathrm{~g}$ of calcium $=1 \mathrm{~mol}$
$8.0 \mathrm{~g}$ of calcium $=1 \mathrm{~mol} \times \frac{(8.0 \mathrm{~g})}{(40 \mathrm{~g})}=0.2 \mathrm{~mol}$.

Question $33 .$
The density of water at room temperature is $1.0 \mathrm{~g} / \mathrm{mL}$. How many molecules are there in a drop of water if its volume is $0.05 \mathrm{~mL}$ ?
Solution:
Volume of a drop of water $=0.05 \mathrm{~mL}$
Mass of a drop of water $=$ Volume $\times$ density $=(0.05 \mathrm{~mL}) \times(1.0 \mathrm{~g} / \mathrm{mL})=0.05 \mathrm{~g}$
Gram molecular mass of water $\left(\mathrm{H}_{2} \mathrm{O}\right)=2 \times 1+16=18 \mathrm{~g}$
$18 \mathrm{~g}$ of water $=1 \mathrm{~mol}$
$0.05 \mathrm{~g}$ of water $=\frac{1 \mathrm{~mol}}{(18 \mathrm{~g})} \times(0.05 \mathrm{~g})=0.0028 \mathrm{~mol}$
No. of molecules present
1 mole of water contain molecules $=6.022 \times 10^{23}$
$0.0028$ mole of water contain molecules $=6.022 \times 1023 \times 0.0028=1.68 \times 10^{21}$ molecules.

Question $34 .$
Calculate the total number of electrons present in $1.6 \mathrm{~g}$ of methane.
Solution:
(i) Molar mass of methane $\left(\mathrm{CH}_{4}\right)=12+4 \times 1=16 \mathrm{~g}$
$16 \mathrm{~g}$ of methane contain molecules $=6.022 \times 10^{23}$
$1.6 \mathrm{~g}$ of methane contain molecule $==\frac{6.022 \times 10^{23}}{(16 \mathrm{~g})} \times(1.6 \mathrm{~g})=6.022 \times 10^{22}$
(ii) Number of electrons in $6.022 \times 10^{22}$ molecules of methane
1 molecule of methane contains electrons $=6+4=10$
$6.022 \times 1022$ molecules of methane contain electrons $=6.022 \times 1022 \times 10=6.022 \times 10^{23}$
 

Question $35 .$
The Vapour Density of a gaseous element is 5 times that of oxygen under similar conditions. If the molecule is triatomic, what will be its atomic mass?
Solution:
Molecular mass of oxygen $=32 \mathrm{u}$
Density of oxygen $=\frac{32}{2}=16 \mathrm{u}$
Density of gaseous element $=16 \times 5=80 \mathrm{u}$
Molecular mass of gaseous element $=80 \times 2=160 \mathrm{u}$
Atomicity of the element $=3$
Atomic mass of the element $=\frac{\text { Molecular mass }}{\text { Atomicity }}=\frac{160}{3}=53.33 \mathrm{u}$.