Question

The composition of an acidic buffer solution of $NaA$ and $HA$ of total molarity $0.29$ having $pH=4.4$ and ${KA}_a=1.8\times {10}^{-5}$ will be:

• A. $0.25M$
• B. $0.20M$
• C. $0.30M$
• D. $0.15M$

Answer 1

Answer: (B)

$0.20M$

Let $a$ $mol$ ${litre}^{-1}$ be concentration of salt, then concentration of acid $=(0.29-a)$

For buffer solution: $pH=-\log K_a+\log \frac{\left[Conjugatebase\right]}{\left[Acid\right]}$

or $4.4=-\log 1.8\times {10}^{-5}+\log \frac{a}{(0.29-a)}$

$\therefore$ $a=0.09M$

$\therefore$ $\left[Salt\right]=0.09M$

$\left[Acid\right]=0.29-0.09=0.20M$

Hence, option $B$ is correct.