Question

The $pH$ of a $0.01M$ $HCN$ solution for which ${pK}_a$ is $4$ is:

• A. $0.47$
• B. $1.2$
• C. $3.0$
• D. $4.0$

Answer 1

Answer: (C)

$3.0$

Given, $p{K}_a=4$

$\therefore K_a=1\times {10}^{-4}$

$\left[H^{+}\right]=\sqrt{K_a.C}=\sqrt{1\times {10}^{-4}\times 0.01}$

$=\sqrt{{10}^{-6}}={10}^{-3}M$

$pH=-\log \left[H^{+}\right]=-\log \left({10}^{-3}\right)=3$

Option C is correct.