Question

If you have 3.50 moles of hydrogen and 5.00 moles of nitrogen to produce ammonia, which element is the reactant in excess?

Answer 1

you need to start by looking at the balanced reaction. Have you got one?

Answer 2

The balanced eqn will be:
N2 + 3H2 = 2NH3. Thus, for every mol of N2 you need 3 mol H2. Which means that for 5 mol N2 you need 15 mol. Thus H2 is the limiting reagent.