According to the equation, the oxidation number of N is change from 0 to -3.
So the equivalent mass of N2 = Molar mass6
So volume occupied by 1 equivalent of N2 at STP = 22.46 L
Similarly, equivalent mass of NH3 = Molar mass3
So volume occupied by 1 equivalent of NH3 at STP = 22.43 L
Again, equivalent mass of H2 = Molar mass2
So volume occupied by 1 equivalent of NH3 at STP = 22.42 L
Thus total volume = (22.46+22.43+22.42) L
=(22.4+2×22.4+3×22.4)6 L=22.4 L