The solubility product constant
The solubility constant (product) (Ks) is
CaF2(s)
Ca2+(aq) + 2F–(aq)
Qs = [Ca2+][F–]2
The form of Qs depends on the balanced equation for the dissolution reaction as the powers associated with the ion concentrations in Q depends on the relative numbers of cations and anions in the solid. As is typical for heterogeneous equilibrium pure solids are assigned a value of 1. This is because increasing the amount of solid in contact with a saturated solution does not affect its equilibrium composition.
While the relative equilibrium concentrations of Ca2+ and F– may differ in two saturated solutions at the same temperature, Qs equals Ks for both.
In contrast the solubility s(CaF2) in two saturated solutions differs if, for example, one of the ions arising from dissolution is present before dissolving occurs.
- the equilibrium constant for the reaction in which an ionic solid dissolves to give its ions.
- much less than 1 for sparingly soluble solids like CaF2.
- related to ion concentrations as the magnitude of the reaction quotient Qs at equilibrium (in a saturated solution) is equal to Ks.
CaF2(s)
Ca2+(aq) + 2F–(aq)Qs = [Ca2+][F–]2
The form of Qs depends on the balanced equation for the dissolution reaction as the powers associated with the ion concentrations in Q depends on the relative numbers of cations and anions in the solid. As is typical for heterogeneous equilibrium pure solids are assigned a value of 1. This is because increasing the amount of solid in contact with a saturated solution does not affect its equilibrium composition.
While the relative equilibrium concentrations of Ca2+ and F– may differ in two saturated solutions at the same temperature, Qs equals Ks for both.
In contrast the solubility s(CaF2) in two saturated solutions differs if, for example, one of the ions arising from dissolution is present before dissolving occurs.