Ksp: Solubility Product, Q vs Ksp and the Common Ion Effect

Ksp is the equilibrium constant for a slightly soluble salt dissolving. It tells you how much dissolves and whether mixing two solutions makes a precipitate. The common ion effect is Le Chatelier's principle applied to it.

Writing Ksp

For A_mB_n(s) ↔ mAⁿ⁺ + nB^m-, Ksp = [Aⁿ⁺]^m[B^m-]ⁿ. The solid is left out of the expression.

Only compare Ksp values directly for salts with the same ion ratio.

Q vs Ksp

Common ion effect

Adding an ion the salt already produces pushes the equilibrium toward the solid, so solubility falls. AgCl is less soluble in 0.1 M NaCl than in pure water. Acidic solutions raise the solubility of salts whose anion is a weak base, such as fluorides and carbonates, because H⁺ removes the anion.

Worked example

AgCl has Ksp = 1.8 x 10^-10.

• In pure water: s = √(1.8 x 10^-10) = 1.3 x 10^-5 M.
• In 0.10 M NaCl: [Cl^-] is about 0.10, so s = Ksp/0.10 = 1.8 x 10^-9 M.
• Solubility drops by a factor of about 7000.

Common traps

• Including the solid in the Ksp expression.
• Forgetting stoichiometric powers, and the factor in 2s, when solving for s.
• Comparing Ksp across different ratios to rank solubility.
• Using the pure water s when a common ion is present.

One-minute recap

Ksp = product of ion concentrations raised to their coefficients. Q > Ksp precipitates. A common ion lowers solubility. Acid raises solubility of salts with basic anions.