Determination of molecular formulas

Exercise 3

    

A monocyclic organic substance has two double bonds and a ketone function. its molar mass is $108\;\frac{g}{mol}$. Find its molecular formula.

General molecular formula: $C_nH_{2n-6}O$ $12n$ $+$ $2n$ $-$ $6$ $ +$ $16$ $=$ $108$ $n$ $=$ $7$ $C_7H_8O$