Reaction of ____ mole of magnesium nitride with excess of water can liberate one mole of ammonia. 
 

Reaction of magnesium nitride with an excess of water${\mathrm{Mg}}_3{\mathrm{N}}_2+6{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{Mg}(\mathrm{OH}{)}_2+2{\mathrm{NH}}_3$

Hence one mole of magnesium nitride on reaction with excess water gives two moles of ammonia.

1 mole of Mg₃N₂ -------2 moles of NH₃

1/2 mole of   Mg₃N₂  -----------1 mole of NH₃