When memorizing the solubility rules, you learned that several inorganic compounds were insoluble in water. The word "insoluble" does not accurately define these specific compounds. These compounds are best described as "sparingly soluble". This means that a small amount of the solid salt will dissolve in solution. Using the knowledge learned in General Equilibrium, you can right an equilibrium constant expression for sparingly soluble compounds. The equilibrium constant, Kc, is given the name solubility product constant, Ksp when referring to sparingly soluble compounds.

Notice that the concentration of barium sulfate is not included in the expression. It is a solid and therefore not included in the expression. This was explained in General Equilibrium.

Solubility problems are solved very similar to the equilibrium problems. Usually, the initial concentration of the ions is zero. Instead of representing the change in concentration as x, the symbol s is used. Please follow the example problem below for a clear explanation.

Example problem: Calculate the solubility of CuI if the Ksp = 5.1 E-12.

Notice that the concentration of CuI is not given. Remember, it is a solid, so its concentration is assumed to be 1 and it is not included in the equilibrium expression.