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Basic Buffer Action

At 25 o C, ...

Question

At $25^{o} C$ , the value of $p K_{b}$ ( $K_{b}$ being the dissociation constant at a base) for ${N H}_{3}$ in aqueous solution is $4.7$ . What is the pH of $0.1 M$ aqueous solution of ${N H}_{4} C l$ with $0.01 M$ ${N H}_{3}$ (approximately)?

8.3

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Solution

The correct option is A $8.3$
Aqueous solution of $N H_{3}$ is $N H_{4} O H$ .

$N H_{4} O H + N H_{4} C l$ will form a basic buffer.

So $p O H = p K_{b} + {log}_{10} \frac{[{N H}_{4} C l]}{[{N H}_{4} O H]}$

$p O H = 4.7 + {log}_{10} (\frac{0.1}{0.01}) = 5.7$

$p H = 14 - p O H = 8.3$

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