Solubility Equilibrium

Solubility Equilibrium Solubility is the ability of a substance to dissolve in water. The solubility is measured in terms of concentration of an ion that is present in a smaller ratio in solution. On the other hand, solubility equilibrium refers to the equilibrium between the dissolved salt (ions) and undissolved salt that usually exists in a saturated solution or a solution of a sparingly soluble salt. The word sparingly soluble salt refers to a salt that is partially (not completely) soluble in water, as results of which, the equilibrium between dissolved ions and undissolved salt is possible. Solubility Product To understand the concept of solubility product, consider the saturated solution of silver chloride (AgCl), where the equilibrium exists between dissolved ions and undissolved silver chloride according to the following equation.

⎯⎯ → AgCl ( s ) ←⎯ Ag + ( aq ) + Cl − ( aq ) ⎯

Since this is an equilibrium reaction, we can write the equilibrium constant expression as

K=

[ Ag + ][Cl − ] [ AgCl ( s)]

The concentrations of solids are either unknown or assumed to be constant. Hence we combine [AgCl(s)] with K and label this constant as Ksp. Thus

Kx[ AgCl ( s)] = K sp = [ Ag + ][Cl − ]

The Ksp is called the solubility product constant or simply solubility product. In general, the solubility product of a compound is the product of molar concentrations of ions raised to the power of their respective stoichiometric coefficients in the equilibrium reaction. In the above example, writing the Ksp expression for AgCl is very simple because only one mole of Ag+ ions and one mole of Cl- ions are formed. If more than one mole of ions are formed, then the things get more complex. Consider the following examples:

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MgI2

⎯⎯ → MgI 2 ( s ) ←⎯ Mg 2 + ( aq ) + 2 I − ( aq ) ⎯

K sp = [ Mg 2+ ][ I − ]2

Ag2SO4

⎯⎯ → Ag 2 SO4 ( s ) ←⎯ 2 Ag + ( aq ) + SO4 2 − ⎯

K sp = [ Ag + ]2 [ SO4 2− ] K sp = [ Al +3 ][OH − ]3

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Al(OH)3

⎯⎯ → Al (OH...