Hydrogen and nitrogen react to form ammonia. The relevant chemical equation can be written as:
`N_2 + 3H_2 -> 2NH_3`
Here 1 mole of nitrogen reacts with 3 moles of hydrogen to form 2 moles of ammonia.
In the first case, excess nitrogen is present. Using stoichiometry,
2 moles ammonia requires 3 moles hydrogen,
1 mole of ammonia will require 3/2 moles of hydrogen
and 10 moles of ammonia will require 3/2 x 10 = 15 moles of hydrogen.
The molar mass of hydrogen is 2 g.
Thus, 15 moles x 2 g/mole = 30 g of hydrogen is required to make 10 moles of ammonia.
Molar mass of nitrogen is 28 g and thus, 56 g of nitrogen contains 56 g / 28 (g/mole) = 2 moles.
Using the well balanced chemical equation for formation of ammonia, 1 mole of nitrogen reacts with 3 moles of hydrogen and thus, 2 moles of nitrogen will react with 2 x 3 = 6 moles of hydrogen.
Hope this helps.