Solubility product

Solubility

• A saturated solution is formed when a solvent has dissolved as much of a solute as it can. Any more solid that is added will not dissolve.

• The maximum amount of solid that will dissolve in a certain solvent is called the solubility.

• Solubility is usually given in gdm^-3 and changes with temperature.

Solubility product

• A sparingly soluble substance is one that does not fully dissolve in a solvent.

• When a sparingly soluble substance is dissolved to give a saturated solution an equilibrium is established.

• E.g. BaSO_4(s) ⇋ Ba²⁺_(aq) + SO₄^2-_(aq)

• Just like for any other equilibrium, a K_c expression can be written.

• E.g. for BaSO₄, K_c = [Ba²⁺_(aq)][SO₄^2-_(aq)]

  • Solids are not included in K_c expressions because their concentrations are constant

• The k_c for a saturated solution of a sparingly soluble salt is called the solubility product (K_sp)

• Therefore for BaSO₄, K_sp = [Ba²⁺_(aq)][SO₄^2-_(aq)]

Techniques and procedures for determining the solubility product of Ca(OH)₂

• Add Ca(OH)₂ to distilled water and leave to equilibrate for 48 hours.

• Filter the solution into a conical flask.

• Pipette 25cm³ portions into separate conical flasks

• Titrate against HCl

• Calculations can be performed to determine the solubility of the salt and then work out the solubility product.

Uses of the solubility product

• Indicate solubility

  • The bigger the K_sp, the more soluble the compound.

• Calculated solubility

• Predict precipitation

  • Precipitation occurs when the ionic product > solubility product

  • The ionic product has the same formula as K_sp but uses the actual concentrations of ions in solution

  • Therefore if the ionic product is greater than the solubility product, the solution will be saturated and a precipitate will form