Solubility product

Solubility product: The product of concentrations of ions in saturated solution of an electrolyte at a given tempera­ture is constant and is known as solubility product. For silver chloride, the equilibrium is formulated as follows.

Undissolved salt ⇋ ions in solution

AgCl_(s) = Ag⁺_(aq)+ Cl _(aq)

K = [Ag⁺][Cl^–]/[AgCl]

The concentration of the undissolved salt is constant (say k) at a particular temperature regardless of the amount of solid silver chloride present. (This is true for any solid that its concentration is independent of the amount of solid), we have:

K = [Ag⁺][Cl^–]/k    OR       K.k = [Ag⁺][Cl^–] = k_sp

The constant k_sp is called as the solubility product.

Common ion effect. If the solution of a strong electro­lyte is added to that of a weak electrolyte having a common ion, the ionization of weak electrolyte is suppressed more and this decrease in the degree of ionization of the weak electrolyte in solution on being mixed with a strong electrolyte containing

a common-ion is termed as common ion effect.

For example ionization of NH₄OH is suppressed by the addition of NH₄CI which provides common NH₄⁺ ion.