22.4Solubility Equilibria

22.4 Solubility Equilibria

Solubility product, ftsp

Solubility product, ftsp is the product of the concentrations of each ion in a saturated solution of a sparingly soluble salt at 298 K, raised to the power of their relative concentrations

For any so called ‘insoluble salt’, a tiny amount of it do dissolve in water
- Solubility product only apply if the solution is in equilibrium with its solid, in other words, the solution is saturated.
- It means that in a saturated solution at 298 K, when the concentrations of the ions are multiplied together, the value is the solubility product.
The higher the value of Ksp, the more soluble the salt is.

Solubility product can be used to predict precipitation:
1. If the product of the concentrations of ions is less than the solubility product, the solution is not saturated, no precipitate will be formed.
2. If the product of the concentrations of ions is equal to the solubility product, the solution is saturated, no precipitate is formed yet.
3. If the product of the concentrations of ions is more than the solubility product, the solution is saturated, precipitation occurs to reduce the concentrations to a value equal to the solubility product

Calculations on solubility product

Common ion effect

Consider the equilibrium of barium sulfate and its ions:
BaSO4(s) ⇌ Ba²⁺(aq) + SO4²⁻(aq)
If sulfuric acid, H2SO4 is added, the concentration of sulfate ions increases and the position of equilibrium shifts to the left. More precipitate is formed and barium sulfate becomes less soluble.

The common ion effect is therefore the reduction in the solubility of a dissolved salt by adding a solution of a compound which has an ion in common with the dissolved salt, this often results in precipitation

The solubility of an ionic compound in aqueous solution containing a common ion is less than its solubility in water

Calculations on common ion effect