Concept preview

What you will be able to do

1. 01Write a Ksp expression from a formula
2. 02Convert between Ksp and molar solubility
3. 03Explain and apply the common-ion effect

The solubility product constant, Ksp, is simply the equilibrium constant for the dissolving of a slightly soluble salt. You write it exactly as you write any equilibrium expression: the product of the dissolved-ion concentrations, each raised to the power of its coefficient, with the pure solid omitted. For silver chloride, $\mathrm{AgCl(s) \rightleftharpoons Ag^+ + Cl^-}$, the expression is $K_{sp} = [\mathrm{Ag^+}][\mathrm{Cl^-}]$. For calcium fluoride, $\mathrm{CaF_2(s) \rightleftharpoons Ca^{2+} + 2F^-}$, the coefficient 2 on fluoride becomes an exponent: $K_{sp} = [\mathrm{Ca^{2+}}][\mathrm{F^-}]^2$.