Chapter 9, Problem 9.1P

Interpretation Introduction

Interpretation:

Addition of acid to buffer solution does not change $\text{pH}$ of solution has to be explained.

Concept Introduction:

Buffers are those that resist $\text{pH}$ change on addition of acid or base and even if it is diluted. It is formed by combination of weak acid and strong base or vice versa. Buffer capacity is defined as the quantity of acid or base added to volume of buffer to bring change in $\text{pH}$.

Explanation of Solution

Addition of acid to a buffer solution does not bring any change in $\text{pH}$ of solution. It is due to buffer capacity that resists change in $\text{pH}$. In accordance to Henderson-Hasselbach equation, $\text{pH}$ is calculated by formula as follows:

  $\text{pH}=p{K}_{\text{a}}+\log \left(\frac{\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}\right)$

The maximum value of $\text{pH}$ is attained when $\text{pH}$ becomes equal to $p{K}_{\text{a}}$ and this can happen only if ratio of $\left(\frac{\left[{\text{A}}^{-}\right]}{\left[\text{HA}\right]}\right)$ is one. At this condition there is no change in $\text{pH}$ of solution.