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2020•Biochemistry and Molecular Biology EducationRequires access

From pKa to the pH of weak base solutions

James P. McEvoy

Open publisher page 4 citations

Abstract

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.

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What this paper is about

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.

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Available abstract

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.

Key concepts: Conjugate, Base (topology), Conjugate acid, Weak base, Dissociation constant, Acid–base reaction, Chemistry, Dissociation (chemistry)

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