Question

The $pH$ of a $0.01MHCN$ solution for which $p{K}_a$ is $4$ is

• A. 0.47
• B. 1.2
• C. 3.0
• D. 4.0

Answer 1

Answer: (C)

Given, $p{K}_a=4$
$\therefore K_a=1\times 10^{-4}$
$\left[H^{+}\right]=\sqrt{K_a\cdot C}$
$=\sqrt{1\times 10^{-4}\times 0.01}$
$=\sqrt{10^{-6}}=10^{-3}M$
$pH=-\log \left[H^{+}\right]$
$=-\log \left(10^{-3}\right)=3$