Question

A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29, find its molecular formula.
[C = 12, H = 1]

Answer 1

% of carbon = 82.76%
% of hydrogen = 100 - 82.76 = 17.24%

Element	% weight	Atomic weight	Relative No. of moles	Simplest Ratio
C	82.76	12	82.76/12 = 6.89	6.89/6.8 = 1 x 2 = 2
H	17.24	1	17.24/1 = 17.24	17.24/6.89 =2.5 x 2 = 5

Empirical formula = C₂H₅
Empirical formula weight = 2 × 12 + 1 × 5 = 24 + 5 = 29
Vapour density = 29
Relative molecular mass = 29 × 2 = 58
N = `"Relative molecular mass"/"Empirical weight"`
= `58/29` = 2
Molecular formula = n x empirical formula
= 2 x C₂H_5
= C₄H₁₀