Solubility Constant

Solubility Some ionic compounds dissolve in water while other will not It is not always all or nothing Others are slightly soluble A fraction of the ionic compound will dissolve in water while some A fraction of the compound will dissociate into ions

Equilibrium The solution of a slightly soluble solid exist in equilibrium Ions are separating and reforming at constant rate Every time a molecule dissolves a new one forms from ions reforming The expression can be written as AB2 ⇄ A + 2B The reaction goes forward and backward at the same rate

Solubility Constant At equilibrium, the concentrations of the free ions are known Mathematical relationship exists between the free ions For slightly soluble solids the relationship exits AB2  A+ + 2B- Ksp = [A+] x [B-]2 Note the coefficient becomes the exponent The value for Ksp is known for most subtances

Value of Ksp There is a direct relationship between Ksp and solubility Ksp > 1 indicates a highly soluble substance Ksp < 1 indicates a slightly soluble substance The smaller the number the lower the solubility

Example What is the concentration of Ca+2 ions when CaCO3 is placed in water if Ksp = 3.3 x 10-9? 3.3 x 10-9 = [Ca+2] x [CO3-2] Since the concentrations of the ions are equal The concentrations can written as X 3.3 x 10-9 = X ∙ X 3.3 x 10-9 = X2 find the square root of 3.3 x 10-9 X = 5.7 x 10-5