Henderson-Hasselbalch Equation and Example

The Henderson-Hasselbalch equation is used to calculate pH of a solution or buffer.
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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

The Henderson-Hasselbalch equation relates pH, pKa, and molar concentration (concentration in units of moles per liter):

apH = pK + log ([A-]/[HA])

[A-] = molar concentration of a conjugate base

[HA] = molar concentration of an 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.

This is a straightforward example because all of the terms are given. 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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Helmenstine, Anne Marie, Ph.D. "Henderson-Hasselbalch Equation and Example." ThoughtCo, Nov. 28, 2022, thoughtco.com/henderson-hasselbalch-equation-and-example-603648. Helmenstine, Anne Marie, Ph.D. (2022, November 28). Henderson-Hasselbalch Equation and Example. Retrieved from https://www.thoughtco.com/henderson-hasselbalch-equation-and-example-603648 Helmenstine, Anne Marie, Ph.D. "Henderson-Hasselbalch Equation and Example." ThoughtCo. https://www.thoughtco.com/henderson-hasselbalch-equation-and-example-603648 (accessed June 7, 2023).