Question

The pH of 0.5 M aqueous solution of $HF$ $\left(K_a=2\times 10^{-4}\right)$ is

• A. 2
• B. 4
• C. 6
• D. 10

Answer 1

Answer: (A)

$K_a=C{\alpha }^2$
$2\times 10^4=0.5\times {\alpha }^2$
$\alpha =2\times 10^{-2}$
$\therefore \left[H^{+}\right]=C\alpha =0.5\times 2\times 10^{-2}=10^{-2}$
$pH=2$