be equal to S. The solubility of salts of weak acids like phosphates increases at lower pH. This is because at lower pH the concentration of the anion decreases due to its protonation. This in turn increase the solubility of the salt so that K sp = Q sp .
We have to satisfy two equilibria simultaneously i.e., K sp = [M + ] [X – ], [X – ] / [HX] = K a /[H + ] Taking inverse of both side and adding we get HX X H HX H X H a a a