Solubility equilibria
Consider dissolving of the slightly soluble ionic solid CaF2 to give its constitutent ions. The change in (Δ, delta) concentration of the ions due to dissolving is related to the molar solubility (s) through the coefficients in the balanced equation.
Qs is the reaction quotient for the dissolving of an ionic solid to its constituent ions.
To calculate Ks from the molar solubility s(CaF2) or vice versa
CaF2(s) 
Ca2+(aq) + 2F–(aq)
Qs = [Ca2+][F–]2
Ca2+(aq) + 2F–(aq)Qs = [Ca2+][F–]2
| s(CaF2) | = | Δ [Ca2+] | = | Δ [F–] |
| 1 | 2 |
Qs is the reaction quotient for the dissolving of an ionic solid to its constituent ions.
Qs = Ks in a saturated solution in the presence of excess solid
Ks is the maximum value for Qs without precipitation of the solid.
Ks is the maximum value for Qs without precipitation of the solid.
To calculate Ks from the molar solubility s(CaF2) or vice versa
- Relate the change in concentration of ions due to dissolving to the experimental molar solubility using the dissolving equation
For a MX2 salt as above [Ca2+] is equal to s, and [F–] is equal to 2s
For an MX salt both ion concentrations are equal to the molar solubility.
- Determine the equilibrium concentration of the ions by adding this change to their initial concentration.
If the only source of an ion is dissolving, the initial concentration is zero. - Substitute the result into the Ks expression.