The solubility constant (product) (Ks) is
The form of the reaction quotient expression 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.
While the relative equilibrium concentrations of Ca2+ and F– may differ in two saturated solutions at the same temperature, Qs equals Ks for both.
For a given solution at a particular temperature, precipitation occurs only if the product of concentrations exceeds Ks at that temperature.
- 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 reaction quotient | Ks = 4 × 10–11 at 25°C |
The form of the reaction quotient expression 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.
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.
Thus if [Ca2+] in a saturated solution of CaF2 is known, [F–] can be calculated using the expression above and Ks.
For a given solution at a particular temperature, precipitation occurs only if the product of concentrations exceeds Ks at that temperature.
Thus a precipitate of CaF2 forms if [Ca2+][F–]2 at room temperature is higher than 4 × 10–11, the magnitude of Ks at that temperature.