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Henderson Hasselbalch Equation and Example

Buffers and the Henderson Hasselbalch Equation


The Henderson Hasselbalch equation can be used to calculate pH.

The Henderson Hasselbalch equation can be used to calculate pH.

David Gould, Getty Images

You can calculate the pH of a buffer solution or the concentration of the acid and base using the Henderson Hasselbalch equation. Here's a look at the Henderson Hasselbalch equation and a worked example that explains how to apply the equation.

Henderson-Hasselbalch Equation

pH = pKa + log ([A-]/[HA])

[A-] = molar concentration of a conjugate base

[HA] = molar concentration of a undissociated weak acid (M)

The equation can be rewritten to solve for pOH:

pOH = pKb + log ([HB+]/[ B ])

[HB+] = molar concentration of the conjugate base (M)

[ B ] = molar concentration of a weak base (M)

Example Problem Applying the Henderson-Hasselbalch Equation

Calculate the pH of a buffer solution made from 0.20 M HC2H3O2 and 0.50 M C2H3O2- that has an acid dissociation constant for HC2H3O2 of 1.8 x 10-5.

Solve this problem by plugging the values into the Henderson-Hasselbalch equation for a weak acid and its conjugate base.

pH = pKa + log ([A-]/[HA])

pH = pKa + log ([C2H3O2-] / [HC2H3O2])

pH = -log (1.8 x 10-5) + log (0.50 M / 0.20 M)

pH = -log (1.8 x 10-5) + log (2.5)

pH = 4.7 + 0.40

pH = 5.1

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