A hydrocarbon contains 10.5 g of carbon per gram of hydrogen. 1 litre of the vapour of the hydrocarb

Question

A hydrocarbon contains 10.5 g of carbon per gram of hydrogen. 1 litre of the vapour of the hydrocarbon at $127^{\circ}C$ and 1 atmosphere pressure weighs 2.8 g. Find the molecular formula of the hydrocarbon.

Professor PC Thomas Classes · Answer

Moles of C = $10.5/12 = 0.875$. Moles of H = $1/1 = 1$. Ratio C:H is 7:8, empirical formula is $C_7H_8$ (mass 92). Molar mass $M = dRT/P = (2.8/1) 	imes 0.0821 	imes 400 / 1 \approx 92~g/mol$. Hence, molecular formula is $C_7H_8$.