The acidity [H⁺] of a weak acid (or weak base) depends upon the degree of ionization. The degree of ionization is linked to acid-base equilibria [local] The ionization constant for an acid (K_a) is a way of expressing the degree of ionization. The larger the K_a the stronger (or more ionized) the acid. Similarly the ionization constant for a base is symbolized as K_b, and is an expression of the strength of the base. The ionization of acetic acid,

has the related equilibrium expression:

A weak acid titrated to half-equivalence would have equal concentrations of the acid [CH₃COOH] and its conjugate base[CH₃COO^-]. This reduces the K_a expression to K_a = [H₃O⁺]. Taking the negative log of both side result is the pK_a = pH.

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