Question

What is the formula of a hydrocarbon composed of $86$% carbon and $14$% hydrogen by weight?

• A. ${CH}_4$
• B. $C_2{H}_4$
• C. $C_2{H}_6$
• D. $C_3{H}_8$
• E. $C_4{H}_6$

Answer 1

The correct option is B $C_2{H}_4$

The hydrocarbon is composed of 86% carbon and 14% hydrogen by weight.
Thus 100 g of hydrocarbon will have 86 g carbon and 14 g hydrogen.
The atomic weights of carbon and hydrogen are 12 g/mol and 1 g/mol respectively.
The mass is divided with atomic weight to get number of moles.
The number of moles of carbon $=\frac{86}{12}=7.2$ moles.

The number of moles of hydrogen $=\frac{14}{1}=14$ moles.
The mole ratio $C:H=7.2:14=1:2$
Hence, the empirical formula of hydrocarbon is $C{H}_2$.
Among the given options, the molecular formula $C_2{H}_4$ satisfies the mole ratio $C:H=1:2$.