The balanced chemical reaction is:
N2 + 3H2 = 2NH3
We are given the amount of the product to be produced. This will be the starting point of our calculations.
17 mol NH3 ( 1 mol N2 / 2 mol NH3 ) = 8.5 mol N2
Therefore, the required N2 to produce the given amount of ammonia is 8.5 mol.