The empirical formula of a compound of molecular mass 120 is ${\mathrm{CH}}_2\mathrm{O}$. The molecular formula of the compound ·

$\frac{120}{30}=4;{\left({\mathrm{CH}}_2\mathrm{O}\right)}_4$

${\mathrm{CH}}_2\mathrm{O}$ has mass $12+2+16=30 g$. 

Molar mass is $120 g/mole$ which is 4 times 30, so 4 times ${\mathrm{CH}}_2\mathrm{O}$ is ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_4$