We know that for every mole of nitrogen consumed, 3 moles of hydrogen are consumed.
- For the nitrogen, this means the reaction can occur 4/1 = 4 times.
- For the hydrogen, this means the reaction can occur 10/3 = 3.33 times.
So, hydrogen is the limiting reactant, and if 10 moles of hydrogen are used, then the reaction will occur 3.33 times.
Hence, this means there is enough nitrogen left for the reaction to occur another 0.67 times, and hence 0.67 moles of nitrogen will be left over.