Solubility Product Constant (Ksp)

Definition

For a salt A_aB_b ⇌ a A⁺ + b B⁻, Ksp = [A⁺]^a [B⁻]^b at equilibrium. Activity may be used for high ionic strength.

Example: AgCl

AgCl(s) ⇌ Ag⁺ + Cl⁻; Ksp = [Ag⁺][Cl⁻] = s × s = s².

If Ksp(AgCl) = 1.8 × 10⁻¹⁰, then s = √Ksp ≈ 1.34 × 10⁻⁵ M.

Notes

• Common ion effect reduces solubility: presence of shared ions shifts equilibrium.
• Complexation and pH changes can alter apparent solubility.
• Use activities for concentrated solutions; γ < 1 lowers effective concentration.