DEFINITION OF SOLUBILITY PRODUCT CONSTANT, Ksp
Consider, for example, the slightly soluble salt CaF2 (s) in equilibrium with water. Its limited dissociation and ionization in water can be expressed as follows:
And its solubility product constant, Ksp, as
Note that [CaF2 (s)] does not appear in the Ksp expression since it represents the undissolved portion and its pure solid. It has no concentration.
Also note that [Ca2+] represents the concentration of Ca2+ ion dissolved in the water, while [F-] represents the F- ion concentration dissolved in the water.
Note also that when a given amount of CaF2 (s) is dissolved in a known amount of water:
where CaF2 (aq) is that portion which is dissolved in water. In other words, whatever the proportion of CaF2 (s) that dissolves in water, the same molar concentration is present as Ca2+ (aq) ion but twice that molar concentration is present as F- (aq) ion.
The smaller the Ksp value, the lower the solubility of the salt. It follows that the Ksp value of a salt can be calculated by determining the solubility of each ion and then using equation 2.10.