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 concentrationsK_a = [H⁺(aq)][A^-(aq)] / [HA(aq)]Substituting mol dm^-3 for each concentration gives (mol dm^-3 × mol dm^-3) / mol dm^-3, so K_a has units of mol dm^-3.The value of K_a measures the extent of dissociation. A weak acid dissociates only partially, so at equilibrium most of it is still present as HA and there are few dissociated ions: K_a is small. The larger the K_a, the greater the extent of dissociation and the stronger the acid. Ethanoic acid has K_a = 1.7 × 10^-5 mol dm^-3, and methanoic acid, with a larger K_a, is the stronger of the two.Strong acids such as HCl, HNO₃ and HBr dissociate completely in water, so they are not given K_a values.Because K_a values are often very small, they are also quoted as pK_a = −log₁₀K_a. A smaller pK_a means a stronger acid. Like every equilibrium constant, K_a changes only with temperature.