Solubility products, Ksp

Calculating solubility product, common ion effect


Solubility product constant (Ksp) is a thermodynamic equilibrium constant that describes the degree to which a sparingly soluble compound dissolves in water. It is equal to the product of the concentrations of the ions raised to the powers of their stoichiometric coefficients in the balanced chemical equation representing the dissolution of the compound. For example, consider the dissolution of a sparingly soluble salt MX at equilibrium:

MX (s) ⇌ M^+ (aq) + X^- (aq)

The solubility product expression for this reaction is:

Ksp = [M^+][X^-]

where [M^+] and [X^-] are the concentrations of the dissolved ions in the equilibrium mixture. The value of Ksp depends only on temperature and is a measure of the maximum amount of dissolved ions that can exist in a saturated solution of the salt.

If the ion concentrations in a particular solution are greater than Ksp, the salt will precipitate out of the solution until it reaches equilibrium again. On the other hand, if the ion concentrations are less than Ksp, the solution is unsaturated and additional salt can dissolve until saturation is reached. The Ksp value allows us to predict the solubility behavior of a salt and also to determine its molar solubility (the concentration of the dissolved ions) at equilibrium.