# Solubility Product

This chapter is part of Pharmaceutical Chemistry (PHR 1601).

The solubility product, Ksp, is the equilibrium constant for a solid substance dissolving in an aqueous solution. It represents the level at which a solute dissolves in solution. The more soluble a substance is, the higher the Ksp value it has.

In order for a salt to precipitate out the product of each of the ions has to exceed the solubility product.

For example,the Ksp of silver sulfate would be the product of 2 ions of silver and one ion of sulfate – Ksp = [Ag+]2[SO42-].

Example

Strontium sulfate, SrSO4, has a solubility product of 3.2 x 10-7 mol2 dm-6.

1. Write the equilibrium equation for the changes that happen in a saturated solution of strontium sulfate in the presence of some solid.

2. Work out the concentration of the dissolved strontium ions in mol dm-3.

Molarity = 5.66*10-4

3. If you added some sodium sulfate solution to a saturated solution of strontium sulfate, what would you expect to happen to the concentration of dissolved strontium ions? Explain your answer.

If there is a saturated solution of strontium sulfate a precipitate will form as soon as a soluble sulfate is added. This is due to the fact that the Ksp is a product of both the cation and the anion and thus when the concentration of one of the ions is increased, the saturation point is reached at a lower concentration of the insoluble salt and therefore this would precipitate out.

4. Suppose the concentration of the sulfate ions in the mixture was 0.50 mol dm-3 (virtually all of which is due to the ions from the sodium sulfate), work out the new concentration of the strontium ions after the addition of the sodium sulfate solution.

Concentration of strontium ions = 6.4*10-4