Chapter 3: Atoms and Molecules- Exercise- 39

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Chapter 3: Atoms and Molecules

Class: IX

Exercise number 40

Question 1

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

Answer 1

Molecular mass of H₂ = 2 × Atomic mass of H

= 2 × 1 = 2u

Molecular mass of O₂ = 2 × Atomic mass of O

= 2 × 16 = 32u

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 = 30u

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

= 2 × 12 + 4 × 1 = 28u

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 masses of Zn = 65u, Na = 23u, K = 39u, C = 12u, and O = 16u.

Answer 2

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 = 62u

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

= 2 × 39 + 12 + 3 × 16 = 138u

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