Question

What will happen to the $pH$ of $\frac{M}{10}$ $NH_{4}CN$ solution on dilution?

• A. Increases.
• B. Decreases.
• C. Remains same.
• D. Initially it increases, then decreases.

Answer 1

Answer: (C)

$NH _4 CN$ is a salt of weak acid and weak base.
To calculate its $pH$, we have,
$pH =\frac{1}{2}\left( pK _{ w }+ pK _{ a }- pK _{ b }\right)$
We can see that $pH$ is independent of concentration. Hence, further dilution would not affect it and thus remains same.