The balanced chemical reaction is:
N2 + 3H2 = 2NH3
We use the initial amount of the limiting reactant which is the nitrogen gas and we relate the substances from the reaction above. We do as follows:
3.42 mol N2 ( 2 mol NH3 / 1 mol N2 ) (17.04 g / mol ) = 116.5536 g NH3 produced