Consider the following equation:

4NH₄ + 5O₂ → 4 NO + 6H₂O

How many moles of water are produced when 2 moles of NH4 react with excess Oxygen gas?

See we have two moles of NH4, so we replace 4 NH4 by 2 NH4.

Now, on balancing the equation, we get 2 NH4 + 3O2 - - - > 2NO + 4 H2O (law of conservation of mass)

thus, 2 moles of NH4 react with 3 moles of oxygen gas to produce 2 moles of NO2 and 4 moles of H2O.

Thus, we get 4 moles of H2O