Determination of molecular formulas

Exercise 2

    

A monocyclic organic substance has only one primary amine function. its molar mass is $85\;\frac{g}{mol}$. Find its molecular formula.

1st method: General molecular formula: $C_nH_{2n+1}N$ $12n$ $+$ $2n$ $+$ $1$ $ +$ $14$ $=$ $85$ $n$ $=$ $5$ $C_5H_{11}N$ 2nd method: General molecular formula:: $C_nH_{2n-1}NH_2$ $12n$ $+$ $2n$ $-$ $1$ $+$ $14$ $+$ $2\cdot 1$ $=$ $85$ $n$ $=$ $5$ $C_5H_9NH_2$ (Good method only for simple substances!)