Introduction to Chemical Equilibrium

Solubility Product

Molecules with predominantly ionic bonding form ions in water. Hydration of ions is a factor in solubility, but compounds range from those with very low solubility to some that are highly soluble in water. For ionic solids, solubility is expressed as a solubility product:

Ksp = [ ion1 ] [ ion2 ]

For example, the solubility product for BaSO4 is:
Ksp = [ Ba++ ] [ SO4= ]
The product must include an exponent for the number of a given ion produced per molecule. For example, Ag2SO4 has the expression:
Ksp = [ Ag+ ]2 [ SO4= ]
If the only source of ions is a single material, the amounts are easily seen from the moles and the formula. In other words, two moles of Na2SO4 will provide two moles (four equivalents) of SO4= and four equivalents of Na+. When there are several sources of ions, the Ksp allows calculation of whether precipitation will occur and of the permissible concentration of one species without precipitation if the other ion concentrations are known.

Continue to ionization of water

23-March-96