Question

A gaseous hydrocarbon contains 85.7% carbon and 14.3% hydrogen. 1 liter of the hydrocarbon weighs 1.26 g at NTP?
Determine the molecular formula of the hydrocarbon.

Answer 1

Carbon : Hydrogen mole ratio = $\frac{85.7}{12}:14.3$

= 7 : 14 = 1 : 2

Empirical formula = $C_n{H}_{2n}$ : empirical formula wt = 14

Given 1 litre at NTP weighs 1.26 g

Therefore 22.4 litres will weigh $\frac{22.4}{1.26}$ = 28 g

n = $\frac{28}{14}$ = 2

so the formula is $C_2{H}_4$