Question

Find the molecular formula of a hydrocarbon having vapour density 15, which contains 20% of Hydrogen.

Answer 1

% of hydrogen = 20%

% of carbon = 100 − 20 = 80%

Element	% Weight	Atomic Weight	Relative No. of Moles	Simplest Ratio
C	80	12	`80/12` = 6.667	`6.667/6.667` = 1
H	20	1	`20/1` = 20	`20/6.667` = 2.99 ≈ 3

Empirical formula = CH₃

Empirical formula weight = 1 x 12 + 1 x 3 = 12 + 3 = 15

Vapour Density = 15

Relative molecular mass = 15 x 2 = 30

n = `"Relative molecular mass"/"Empirical Weight" = 30/15 = 2`

Molecular formula = n x empirical formula

 = 2 x CH₃

 = C₂H₆