From the heat of formation of solid NH4Cl (-75.2 kcal/mol) and gaseous NH3 (-11.0), the bond dissociation energies of H2 (104.2) and Cl2 (58.2), the ionization potential of atomic hydrogen (313.4), and the electron affinity of atomic chlorine (-83.4), calculate the gasphase proton affinity of NH3. The lattice energy of NH4Cl may be estimated from Kapustinskii's formula using rN-Cl = 3.50 Å.