# Henderson-Hasselbalch Equation and Example

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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.

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