Intermediate Topic 480 of 1102 Kinetics & Equilibrium
Solubility Product
Ksp is the equilibrium constant for dissolving a sparingly soluble ionic solid.
Practise Kinetics & Equilibrium All Kinetics & Equilibrium topics
In-depth explanation of Solubility Product
The solubility product Ksp is the equilibrium constant for the dissolution of a sparingly soluble ionic solid into its ions in a saturated solution. For a salt such as AgCl the expression is Ksp = [Ag+][Cl-], with the concentration of each ion raised to its stoichiometric coefficient. A smaller Ksp means that less of the solid dissolves at equilibrium, so the solid is less soluble. For salts whose formula contains several ions of the same type, the stoichiometric powers must be included, so Ca3(PO4)2 has Ksp = [Ca2+]^3[PO4^3-]^2. Solubility can be calculated from Ksp by solving the equilibrium expression for the ion concentrations.
Going deeper: Solubility Product
Key ideas: Molar solubility s: for AB, Ksp = s^2; for AB2, Ksp = 4s^3; for A2B3, Ksp = 108 s^5. Compare solubilities by Ksp only when salts have the same formula type. Solubility in g per L equals s times molar mass. Ksp changes with temperature but usually increases for dissolution that absorbs heat.
Common mistakes and connections: Students compare Ksp values directly for salts of different stoichiometry, or forget the exponents in the expression. Another error is including the solid, which has constant activity. Ksp applies only to sparingly soluble salts and is modified in the presence of common ions, complexing agents and changes in pH, since hydroxides and carbonates dissolve in acid. Related to precipitation and the common ion effect.
Visualization of Solubility Product
Molar solubility from Ksp (1.8e-10, 1.1e-10, 3.9e-11, 9.8e-9): salts with ions in a 1:2 ratio, like CaF2 and PbI2, dissolve more than Ksp alone suggests. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For CaF2 with Ksp = 3.9 x 10^-11, the molar solubility is about 2.1 x 10^-4 M.
Frequently asked questions about Solubility Product
What is Solubility Product?
Ksp is the equilibrium constant for dissolving a sparingly soluble ionic solid.
Is Solubility Product a beginner, intermediate or advanced topic?
Solubility Product is a intermediate topic in Kinetics & Equilibrium. It builds on the vocabulary of beginner topics in this area.
What should I learn before Solubility Product?
Study these earlier topics in Kinetics & Equilibrium first: Contact Process, Ostwald Process, Equilibrium and Free Energy.
What comes after Solubility Product?
The next topics in Kinetics & Equilibrium are Common Ion Effect and Precipitation Prediction.
Can I practise Solubility Product with quizzes?
Yes. The interactive lesson includes a self-check question on Solubility Product, and the Kinetics & Equilibrium quiz draws on every topic in this area.
Where Solubility Product fits in Kinetics & Equilibrium
Solubility Product is topic 61 of 91 in Kinetics & Equilibrium.