The subject is stoichiometry. how many moles of nh3 can be produced from 12.0mol of h2 and excess n2?

First,  write the balanced chemical equation involved. Since the reaction is synthesis of NH3, N2 + 3 H2 -> 2 NH3. 
Since N2 is excess, H2 is limiting and we, therefore, use its moles to determine the amount of NH3 produced. In the equation, there are 2 moles of NH3 produced over 3 moles of H2 reactant. Thus, we use this ratio: 
12.0 mol H2 ( 2 mol NH3 / 3 mol H2 ) = mol NH3