Answer :
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
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
4 NH4 + 5 O2 → 4 NO + 6 H2O
(2 mol NH4) x (6 mol H2O / 4 mol NH4) = 3 mol H2O