# What is the Henderson-Hasselbalch equation? How is it used in chemistry?

May 14, 2016

$p H = p {K}_{a} + \log \left({n}_{\text{Salt}} / {n}_{A c i d}\right)$

#### Explanation:

The Handerson Hasselbalch equation is used to determine the pH of a buffer solution.

$p H = p {K}_{a} + \log \left({n}_{\text{Salt}} / {n}_{A c i d}\right)$

A buffer solution consists of a weak acid (or weak base) and its conjugate salt. Its function is to resists pH changes upon the addition of small amounts of either acids or bases. It acts as an acid in the presence of a base and acts as a base in the presence of an acid.

The buffer solution is used to maintain a stable pH.

Example

FInd the pH of a buffer solution that is prepared by dissolving 0.20 mol. of acetic acid ($C {H}_{3} C O O H$) and 0.10 mol. of sodium acetate ($C {H}_{3} C O O N a$) in enough water to make 1.00 L solution.

${K}_{a} \text{ for acetic acid } = 1.8 \cdot {10}^{-} 5$

$p H = p {K}_{a} + \log \left({n}_{\text{Salt}} / {n}_{A c i d}\right)$

$p H = 4.74 + \log \left(\frac{0.10}{0.20}\right)$

$p H = 4.44$