# How to determine the molecular formula based on molecular weight, amount of substance, and partial masses of some, but not all contained elements?

I am working on a chemistry assignment and can’t figure out part of the problem.

$0.2\,\mathrm{mol}$ of a compound with a molecular weight of $82$ contain $9.6\,\mathrm{g}$ of carbon, $1.2\,\mathrm{g}$ of hydrogen and the rest is nitrogen.

The question is asking for the empirical and molecular formula which I can solve, but how do you determine the amount of nitrogen?

First I assumed a $100\,\mathrm{g}$ sample but that didn’t get me anywhere. The empirical formula answer is supposed to be $\ce{C2H3N}$ but I couldn’t achieve that. Unless the book is wrong.

If you have $0.2\ \mathrm{mol}$ of a compound with $M_\mathrm{r}=82$, then you have $16.4\ \mathrm{g}$ of material. If there is only carbon, hydrogen and nitrogen present, and the carbon and hydrogen total up to $10.8\ \mathrm{g}$, then the amount of nitrogen must be $16.4\ \mathrm{g}-10.8\ \mathrm{g}=5.6\ \mathrm{g}$.