The acid and base dissociation constants

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 acid dissociation constant higher than one , 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 acid dissociation constant lower than one . 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.

Figure 1. Acid and base dissociation constant equations.

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

Kw = Ka x Kb K W equals K A times K B

Where Kw 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.

pKa and pKb p k a and p k b

Ka and Kb k a and K B 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] p k a is the inverse logarithm of k a. The formula is minus log K A .

pKb is the inverse log of Kb. The formula is -log[Kb] p k b is the inverse logarithm of k b. The formula is minus log k b .