Shown here is a case of the dissolution of an 'insoluble' salt in a solution of known ionic strength. The ionic strength is known because of the presence of a soluble salt. Consider first the molar solubility without taking activity effects into account. Now we account for the lower activity due to the presence of ions in solutions.
Sodium sulfate is a soluble salt.
Now one solves for the molar solubility of CaF 2 . Again, the solubility product equation and the solubility table are used. The following example is more complicated than that shown the above. In particular, the ionic strength changes with the dissolution of the salt. This is important when the salt is relatively soluble.