25.1.9—Ksp from concentrations and vice versa

Let the molar solubility be s, express each ion concentration as its stoichiometric multiple of s, then substitute into Ksp. Conversely, solve the expression for s from measured ion concentrations.

Use the correct power and units; for salts producing several ions, the solubility is not equal to every ion concentration.

For AgCl, if s=1.0×10⁻⁵ mol dm⁻³ then Ksp=s²=1.0×10⁻¹⁰. For CaF₂, [F⁻]=2s, so Ksp=4s³.

Do not use s² for every salt or forget that common ions change the initial concentration before equilibrium.