Question

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?

Answer 1

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=91.43/12:8.57/1=7.62:8.57=1:8.57/7.62=1:1.125=8:9`

So empirical formula of the hydrcarbon is `C_8H_9`

Let its MF be `(C_8H_9)_n`

As per this formula its molar mass becomes `(8*12+9*1)n=105n" g/mol"`

Its molar has been given `105" g/mol"`

So `105n=105=>n=1`

Hence MF of the HC is `C_8H_9`