Question

Calculate the volume, mass and number of molecules of carbon dioxide when 104 g of acetylene (C₂H₂ ) burnt in air. (Atomic masses of C = 12 U, H = 1 U, O = 16 U).

Answer 1

The balanced chemical equation is 

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O 

2 × (2 × 12U + 2 × lU) + 5 × (2 × 16U) → 4 × (12U + 2 × 16U) + 2(2 × 1U + 16U) 

52 g + 160g → 176 g + 36 g 

52 g of acetylene when burnt in air produces 176 g carbon dioxide. 

The amount of carbon dioxide produced when 104 g acetylene burnt = (104/52) x 176 = 352 g

At S.T.P. 1 gram molar mass of any gas occupies 22.4 litres.

So 44g carbon dioxide occupies 22.4 litres volume. 

The volume occupied by 352 g carbon dioxide = (352/44)× 22.4 = 8 × 22.4 = 179.2 litres. 

44 g of carbon dioxide i.e., 1 mole of CO₂ contains 6.02 × 10²³ molecules. So the number of molecules present in 352 g of carbon dioxide 352 

= (352/44)× 6.02 × 10²³ = 8 × 6.02 × 10²³ 

= 4.816 × 10²⁴ molecules.