Suppose there is 1.00 L of an aqueous buffer containing 60.0 mmol of formic acid (pKa=3.74) and 40.0 mmol of formate. Calculate the pH of this buffer.

pH = 3.56

Explanation:

The pH of the buffer producing from the mixture of formic acid and formate ion can be found using H-H equation:

pH = pka + log [A⁻] / [HA]

pH = 3.74 + log [Formate] / [formic acid]

Where [] represents molar concentrations -or moles- of formate and formic acid in the solution.

Replacing knowing moles of formic acid = 0.0600 and moles formate = 0.0400:

pH = 3.74 + log [Formate] / [formic acid]

pH = 3.74 + log [0.0400] / [0.0600]