Page No 40: - Chapter 3 - Atoms & Molecules - Ncert Solutions class 9 - Science

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Question 1:

Calculate the molecular masses of H₂, O₂, Cl₂, CO₂, CH₄, C₂H₆, C₂H₄, NH₃, CH₃OH.

Answer:

Molecular mass of H₂ = 2 × Atomic mass of H

= 2 × 1

= 2 u

Molecular mass of O₂ = 2 × Atomic mass of O

= 2 × 16

= 32 u

Molecular mass of Cl₂ = 2 × Atomic mass of Cl

= 2 × 35.5

= 71 u

Molecular mass of CO₂ = Atomic mass of C + 2 × Atomic mass of O

= 12 + 2 × 16

= 44 u

Molecular mass of CH₄ = Atomic mass of C + 4 × Atomic mass of H

= 12 + 4 × 1

= 16 u

Molecular mass of C₂H₆ = 2 × Atomic mass of C + 6 × Atomic mass of H

= 2 × 12 + 6 × 1

= 30 u

Molecular mass of C₂H₄ = 2 × Atomic mass of C + 4 × Atomic mass of H

= 2 × 12 + 4 × 1

= 28 u

Molecular mass of NH₃ = Atomic mass of N + 3 × Atomic mass of H

= 14 + 3 × 1

= 17 u

Molecular mass of CH₃OH = Atomic mass of C + 4 × Atomic mass of H + Atomic mass of O

= 12 + 4 × 1 + 16

= 32 u

Question 2:

Calculate the formula unit masses of ZnO, Na₂O, K₂CO₃, given atomic masses of Zn = 65 u, Na = 23 u, K = 39 u, C = 12 u, and O = 16 u.

Answer:

Formula unit mass of ZnO = Atomic mass of Zn + Atomic mass of O

= 65 + 16

= 81 u

Formula unit mass of Na₂O = 2 × Atomic mass of Na + Atomic mass of O

= 2 × 23 + 16

= 62 u

Formula unit mass of K₂CO₃ = 2 × Atomic mass of K + Atomic mass of C + 3 × Atomic mass of O

= 2 × 39 + 12 + 3 × 16

= 138 u