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Standard XII

Chemistry

Basic Buffer Action

Calculate the...

Question

Calculate the change in pH of one litre buffer solution containing $0.10 m o l e$ each of ${N H}_{3}$ and ${N H}_{4} C l$ upon addition of (i) $0.02$ mole of dissolved gaseous $H C l$ , (ii) $0.02 m o l e$ of dissolved $N a O H$ . Assume no change in volume $K_{b}$ for ${N H}_{3} = 1.8 \times 10^{- 5}$ )

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Solution

$p O H$ of ${N H}_{3}$ and ${N H}_{4} C l$ buffer
$p O H = - log K_{b} + log \frac{[S a l t]}{[B a s e]}$
$\Rightarrow p O H = - log 1.8 \times 10^{- 5} + log \frac{0.1}{0.1} = 4.75$
$p H = 14 - 4.75 = 9.25$
First Case:
${N H}_{3} + H C l ⟶ {N H}_{4} C l$
$[S a l t] = (0.1 + 0.02) = 0.12 M$
$[B a s e] = (0.1 - 0.02) = 0.08 M$
$p O H = - log K_{b} + log \frac{0.12}{0.08} = 4.926$
$p H = (14 - 4.926) = 9.074$
$Δ p H = (9.25 - 9.074) = 0.176$

Second case:
${N H}_{4} C l + N a O H ⟶ {N H}_{3} + N a C l$
$[S a l t] = (0.1 - 0.02) = 0.08 M$
$[B a s e] = (0.1 + 0.02) = 0.12 M$
$p O H = - log K_{b} + log \frac{0.08}{0.12} = 4.574$
$p H = (14 - 4.574) = 9.426$
$Δ p H = (9.426 - 9.25) = 0.176$

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