From pKa to the pH of weak base solutions

Abstract

Undergraduate biochemistry students frequently find the quantitative treatment of weak acids and bases troublesome. Given the pK_a of a weak acid HA, for instance, many students struggle to calculate the pH of a solution of the conjugate base A^- at concentration C, pH(A^- , C). The traditional method involves calculating the base dissociation constant K_b and the artificial quantity pOH before reaching pH, but these steps increase the risk of mistakes and provide little insight into acid-base equilibria. The alternative method presented here allows students to calculate the pH of a weak base solution from the pK_a of its conjugate acid without calculating K_b and pOH, using a memorable relationship: pH(HA, C) + pH(A^- , C) = pK_a + 7.

Keywords: acids and bases; aqueous solutions; biological chemistry; calculations.