Relationships between solubility, ion concentrations and Ks
Ionic solids dissolve to give rise to ions in solution as shown for CaF2 below. CaF2 is a slightly soluble solid; therefore this reaction is reactant-favoured. The equilibrium constant expression (Ks) for the dissolving reaction is also shown.
The amount in moles of dissolving reaction that occurs per litre of solution is equal to the molar solubility (s) in mol L–1 of the solid.
The molar solubility is in turn related to the concentrations in mol L–1 of the ions in solution arising in the dissolving reaction. Provided these ions arise exclusively from dissolving, their concentrations are equal to their coefficient in the balanced equation times the molar solubility.
For CaF2 dissolved in water where there is no other source of Ca2+ or F–:
[Ca2+] = s and [F–] = 2s
For solutions having a second source of either of these ions, the concentration from the second source must be added to that arising from dissolving.
Using the relationships between ion concentrations and solubility given above,
CaF2(s) 
Ca2+(aq) + 2F–(aq)
Ks(CaF2) = [Ca2+][F–]2
Ca2+(aq) + 2F–(aq)Ks(CaF2) = [Ca2+][F–]2
The amount in moles of dissolving reaction that occurs per litre of solution is equal to the molar solubility (s) in mol L–1 of the solid.
The molar solubility is in turn related to the concentrations in mol L–1 of the ions in solution arising in the dissolving reaction. Provided these ions arise exclusively from dissolving, their concentrations are equal to their coefficient in the balanced equation times the molar solubility.
For CaF2 dissolved in water where there is no other source of Ca2+ or F–:
[Ca2+] = s and [F–] = 2s
For solutions having a second source of either of these ions, the concentration from the second source must be added to that arising from dissolving.
Using the relationships between ion concentrations and solubility given above,
Ks can be calculated if the molar solubility is known.
The concentrations of the ions in a saturated solution can be calculated if Ks is known.
For a water solution the expressions above are simply substituted into Ks.