The acid dissociation constant (Ka) and base dissociation constant (Kb) values represent how easily an acid or base dissociates into ions, in a solution.

Strong acids and strong bases fully dissociate. They possess a Ka > 1, meaning that dissociation is carried out completely. There is no mix of undissociated molecules and ions at equilibrium, only ions.

Weak acids have a Ka < 1. At equilibrium, in solution, there is a mixture of complete molecules and dissociated ions. ICE tables can be used to calculate ratios of reactants and products at equilibrium.

Formulae

Formula for acid dissociation constant Ka and base dissociation constant Kb. Ka equals the concentration of conjugate base times the concentration of hydronium ions divided by the concentration of acid molecules. Kb equals the concentration of conjugate acid times the concentration of hydroxide ions divided by the concentration of base molecules.

For acid and its conjugate base, or a base and its conjugate acid:

Kw = Ka x Kb

Where Kw is the ion constant for water.

Polyprotic acids and bases

These species can donate or receive more than one proton, e.g. sulfuric acid. The second dissociation constant is usually extremely low.

pKa and pKb

Ka and Kb can be converted into easier forms to work with, by finding their inverse logs.

pKa is the inverse log of Ka. The formula is -log[Ka]

pKb is the inverse log of Kb. The formula is -log[Kb]