Vapor density of a compound is 39. It contains carbon and hydrogen atoms in the ratio 1 : 1. Its molecular formula is

Vapor density = 39

Molecular weight=2$\times$vapor density

$=2\times 39=78$

C&H ratio =1:1

Empirical formula = CH

Empirical formula weight =  12 +1 =13

$\mathrm{n}=\frac{\mathrm{M}.\mathrm{wt}}{\mathrm{E}.\mathrm{F}. \mathrm{wt}}=\frac{78}{13}=6$

$\mathrm{M}.\mathrm{F}={(\mathrm{E}.\mathrm{F})}_{\mathrm{n}}={\left(\mathrm{CH}\right)}_6={\mathrm{C}}_6{\mathrm{H}}_6$