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 pKa 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 Kb 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 pKa of its conjugate acid without calculating Kb and pOH, using a memorable relationship: pH(HA, C) + pH(A−, C) = pKa + 7.