Now lets look at the energies in the exponent. As always, the energy scale is relative. From whatever zero point you measure your energy to make a Ag vacancy or a Cl vacancy, you must subtract the energy of the AB molecule as measured in that system. Note that as in treatment given before , the mass action law alone does not specify the vacancy concentration, only their product. OK , we do have a problem, but not with the zero.
Lets look at equilibrium another way.
As shown above for Frenkel defects, it is often advisable not to use the mass action law directly, but to go one step back and use the equilibrium condition for the chemical potentials.