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Basic Buffer Action

A buffer solu...

Question

A buffer solution is prepared bymixing $50 m L$ of $0.5$ molar ammonia solution, $40 m L$ of $0.4$ molar ${N H}_{4} C l$ solution and $10 m L$ of distilled water. Calculate the pH of the buffer.
$K_{b} ({N H}_{3}) = 1.8 \times 10^{- 8}$ at $298 K$ .

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Solution

Total volume $= 50 + 40 + 10 = 100 m L$
Molarity of ${N H}_{3} = \frac{0.5 \times 50}{100} = 0.25 M$
Molarity of ${N H}_{4} C l = \frac{0.4 \times 40}{100} = 0.16 M$
For basic buffer:
$p {O H} = p K_{b} + log \frac{[S a l t]}{[B a s e]}$
$p O H = - l o g (K_{b}) + log \frac{[S a l t]}{[B a s e]}$
$p O H = - l o g (1.8 \times 10^{- 8}) + log \frac{[0.16]}{[0.25]}$ = $9.45$

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