Solubility of a solid electrolyte

Consider an equilibrium between a crystalline salt (or other kind of ionic solid) and a solution containing the solvated ions  

Here v+ and v- are the numbers of cations and anions in the formula unit of the salt, and z+ and z are the charge numbers of these ions. The solution in equilibrium with the 

Thermodynamics and Chemistry, second edition, version 3 20 by Howard DeVoe. Latest version www.chem.umd.edu/thermobook 

We can readily derive a relation between Ks and the molalities of the ions in the saturated solution by treating the dissolved salt as a single solute substance, B. We write the equiUbrium in the form B (s) B(sln), and write the expression for the solubility product as a proper quotient of activities  

From Eq. 10.3.16 on page 293, we have Um,B = y im+/m°y+ (tn-/m°y-. This expression is valid whether or not the ions M + and X are present in solution in the same ratio as in the solid salt. When we replace Um,B with this expression, and replace flg with Eg (Table 9.5), we obtain 

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