NH4X+NaNO2⟶NH4NO2+NaX
Find out X.
Cl
This is the laboratory method of preparing N2 gas:
Ammonium chloride and sodium nitrite are mixed together and heated.
NH4Cl+NaNO2⟶NH4NO2+NaCl
Ammonium nitrite is formed and it is unstable.
NH4NO2⟶N2+2H2O
So it decomposes to give nitrogen.