Solubility Equilibrium

Consider the equilibrium between solid silver chloride and Ag⁺ and Cl^- in a saturated aqueous solution:

The equilibrium constant expression is

The solubility product constant allows us to predict the degree of completeness of precipitation reactions. Whenever the product of the concentrations (each raised to the appropriate power) exceeds Ksp, the salt will precipitate until the concentration product equals Ksp.

Example 1

Compute the solubility of Pb(IO₃)₂ in pure water,

                                  [Pb²⁺]/M                 [IO3^-]/M

Initial                           0                                0

equilibrium                  x                                2x

from which we find x = [Pb²⁺] = 4.0 × 10^-5 M, [IO₃^-] = 8.0 × 10^-5 M. The solubility of Pb(IO₃)₂ thus is 4.0 × 10^-5 mol/L.