The Acid Dissociation Constant Ka

A weak acid has the following equilibrium in aqueous solution:HA _(aq) ⇌ H⁺_(aq) + A^-_(aq)The expression for the acid dissociation constant isThe units can be found by substituting units for the concentrationsWritten out, the expression for a weak acid HA isK_a = [H⁺(aq)][A^-(aq)] / [HA(aq)]Substituting units gives (mol dm^-3 × mol dm^-3) / mol dm^-3, so the units of K_a are mol dm^-3.K_a measures the extent to which a weak acid dissociates. A larger K_a means the equilibrium lies further to the right, more H⁺ ions are released and the acid is stronger. For example, methanoic acid (K_a = 1.6 × 10^-4 mol dm^-3) is a stronger acid than ethanoic acid (K_a = 1.7 × 10^-5 mol dm^-3).Strong acids such as hydrochloric acid dissociate completely, so K_a is not used for them. Like every equilibrium constant, K_a changes only with temperature.Because K_a values are often very small, they are frequently converted to pK_a:pK_a = -log₁₀K_aThe smaller the pK_a, the stronger the acid.