Question

Useful buffer range of weak acid $HA(K_a={10}^{-5})$ is:

• A. $5$ to $7$
• B. $4$ to $6$
• C. $3$ to $5$
• D. none of these

Answer 1

The correct option is B $4$ to $6$

$K_a={10}^{-5}$
$pKa=-lo{g}_{10}{K}_a=-lo{g}_{10}{10}^{-5}=5$
$pH=p{K}_a+\log \frac{\left[\text{Conjugate Base}\right]}{\left[Acid\right]}$
When, $\frac{\left[\text{Conjugate Base}\right]}{\left[Acid\right]}=\frac{1}{10}$
$pH=5+\log \frac{1}{10}$
$pH=5+\log 0.1$
$pH=5-1$
$pH=4$
When, $\frac{\left[\text{Conjugate Base}\right]}{\left[Acid\right]}=\frac{10}{1}$
$pH=5+\log \frac{10}{1}$
$pH=5+\log 10$
$pH=5+1$
$pH=6$
Useful buffer range of weak acid $HA$ is 4 to 6.