A measure of an ionic compound's water solubility is the solubility product.
The solubility product is given by the product of the concentrations of the ions in the solution, each raised to the power of their stoichiometric coefficient. For example, consider the dissolution of a generic salt, AB, in water:
AB(s) ⇌ A+(aq) + B-(aq)
The solubility product constant, Ksp, can be written as:
Ksp = [A+][B-]
where [A+] and [B-] are the molar concentrations of A+ and B- ions, respectively. Note that the concentrations are raised to the power of their stoichiometric coefficients, which are both 1 in this case.
The solubility product of an ionic compound can be calculated from the solubility of the compound in water, which is defined as the maximum amount of the compound that can be dissolved in a given amount of water at a particular temperature. Solubility data for many ionic compounds are available in tables or can be experimentally determined.
Note: The solubility product is dependent on temperature and pressure, and changes in these conditions can affect the solubility of the compound and therefore its solubility product.