Solubility Product Constant

Learning Goals

You will learn how to write the solubility product constant for some salts, become familiar with the relative values of some Ksp's, and learn the difference between solubility and solubility product.


When a salt has dissolved to the greatest extent possible, the solution is said to be saturated with respect to that salt. If the salt is the hypothetical AB salt, when the salt dissolves in water we can write the equilibrium expression as AaBb --> aA+ + bB- and we can write an equilibrium constant in terms of the product ions as K = [A+]a[B-]b. This product of concentrations raised to the appropriate value is a constant [it is a function of temperature as are all equilibrium constants]. If we know the equilibrium constant and one of the concentrations we can calculate the other concentration. This equilibrium constant is called the solubility product constant and is represented by the symbol Ksp. The solubility product constant for the salt BiI3 for example, would be Ksp = [Bi3+][I-]3. Table 19.2 gives some solubility product constants at 25 oC for some insoluble salts. The solubility product is an equilibrium constant whereas the solubility is the concentration achievable by a salt in a given quantity of solvent.

Review Question

Write the solubility product constants for the following salts

  1. BaSO4
  2. Al(OH)3
  3. Bi2S3
  4. Pb3(PO4)2


Web Author: Dr. Leon L. Combs
Copyright 2000 by Dr. Leon L. Combs - ALL RIGHTS RESERVED