In order to react with 3.15 g of N2, we would need 6.3 grams of H2. This is because the balanced chemical equation for the reaction of nitrogen (N2) and hydrogen (H2) to form ammonia (NH3) is:
N2 + 3H2 → 2NH3
So for every 1 mole of N2, we need 3 moles of H2. Since the molar mass of N2 is 28.02 g/mol and that of H2 is 2.02 g/mol, we can use the following conversion to find the number of grams of H2 needed:
3.15 g N2 x (3 moles H2/1 mole N2) x (2.02 g H2/1 mole H2) = 6.3 g H2