Consider the reaction: 2 NO (g) + 5 H2 (g) → 2 NH3 (g) + 2 H2O (g) A reaction mixture initially contains 5 moles of NO and 10 moles of H2. Which set of amounts best represents the mixture after the reactants have reacted as completely as possible?

2 NO (g) + 5 H2 (g) → 2 NH3 (g) + 2 H2O (g)

From the reaction above, 2 moles of NO requires 5 moles of H2 to obtain 2 moles of NH3 and 2 moles of H2O

If all the 5 moles of NO was used, it would require;

2 moles of NO = 5 moles of H2

5 moles of NO = x

x = 12.5 moles of H2

However there is only 10 moles of H2. This makes it the limiting reactant.

When the 10 moles of H2 are used;

5 moles of H2 = 2 moles of NO

10 moles of H2 = x

x = 4 moles of NO

This means 4 moles of NO and 10 moles of H2 are used.

Only 1 mole of NO would remain.