Question

Buffer capacity is maximum when the ratio , $\left(\frac{\text{[Anion of salt]}}{\text{[Acid]}}\right)$ is :

• A. 1 : 2
• B. 1 : 10
• C. 10 : 1
• D. 1 : 1

Answer 1

The correct option is D 1 : 1

Buffer capacity :
It is defined as the moles of a strong acid or strong base required to change the pH of 1 L of a buffer by one unit.

Mathematically, it is the ratio of the small amount of acid or base added to the change in pH caused in the buffer.

The buffering region for a buffer is within one $pH$ unit above or below the pKa of the weak acid composing it i.e. buffer range is within $(p{K}_a\pm 1)$.

A buffer has its maximum buffering capacity (maximum ability to resist $pH$ change) when the $pH$ of the solution equals the $p{K}_a$.
$pH=p{K}_a+log\left(\frac{\left[\text{Anion of salt}\right]}{\left[Acid\right]}\right)$
$\left(\frac{\text{[Anion of salt]}}{\text{[Acid]}}\right)=1log\left(\frac{\text{[Anion of salt]}}{\text{[Acid]}}\right)=0\Rightarrow pH=p{K}_a$