Question

A gaseous hydrocarbon contains 82.76% of carbon. Given that its vapour density is 29, find its molecular formula.

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	`6.89/6.89` = 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 x 12 + 1 x 5 = 24 + 5 = 29

Vapour Density = 29

Relative molecular mass = 29 x 2 = 58

N = `"Relative molecular mass"/"Empirical weight" = 58/29 = 2`

Molecular formula = n × empirical formula

= 2 x C₂H₅

= C₄H₁₀