Solubility
When a solute forms a saturated solution in a solvent, the concentration of the solute, at a given temperature, is determined by the equilibrium constant at that temperature.
The activity of a pure substance in the solid state is one, by definition, so the expression simplifies to
If the solute does not dissociate the summation is replaced by a single term, but if dissociation occurs, as with ionic substances
For example, with Na2SO4 m1=2 and m2=1 so the solubility product is written as
Concentrations, indicated by, are usually used in place of activities, but activity must be taken into account of the presence of another salt with no ions in common, the so-called salt effect. When another salt is present that has an ion in common, the common-ion effect comes into play, reducing the solubility of the primary solute.
Read more about this topic: Equilibrium Chemistry