The acid and base dissociation constants

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

Strong acids and strong bases fully dissociate. They possess a K_a > 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 K_a < 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

Figure 1. Acid and base dissociation constant equations.

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

K_w = K_a x K_b

Where K_w 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.

pK_a and pK_b

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

pK_a is the inverse log of K_a. The formula is -log[K_a] .

pK_b is the inverse log of K_b. The formula is -log[K_b] .