# A hydrocarbon contains a benzene ring. It is made up of 91.43% carbon and its molar mass is about 105g/mol. What is the molecular formula of this hydrocarbon?

Oct 31, 2016

The given hydrocarbon contains

C->91.43% and H->8.57%

Dividing the percentage composition of constituent elements with respective atomic mass and takig their ratio we get the ratio of number of atoms of Carbon and Hydrogen as

${N}_{C} : {N}_{H} = \frac{91.43}{12} : \frac{8.57}{1} = 7.62 : 8.57 = 1 : \frac{8.57}{7.62} = 1 : 1.125 = 8 : 9$

So empirical formula of the hydrcarbon is ${C}_{8} {H}_{9}$

Let its MF be ${\left({C}_{8} {H}_{9}\right)}_{n}$

As per this formula its molar mass becomes $\left(8 \cdot 12 + 9 \cdot 1\right) n = 105 n \text{ g/mol}$

Its molar has been given $105 \text{ g/mol}$

So $105 n = 105 \implies n = 1$

Hence MF of the HC is ${C}_{8} {H}_{9}$