Ionic Equilibrium Question 6
Question 6 - 2024 (30 Jan Shift 2)
The $pH$ of an aqueous solution containing $1 M$ benzoic acid $\left(pK _a=4.20\right)$ and $1 M$ sodium benzoate is 4.5 . The volume of benzoic acid solution in $300 mL$ of this buffer solution is __________ $mL$.
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Answer (100)
Solution
| $1 M$ Benzoic acid | $+1 M$ Sodium Benzoate | |
|---|---|---|
| Milimole | $\left(V _a ml\right)$ | $\left(V _s ml\right)$ |
| $V _a \times 1$ | $V _s \times 1$ |
$pH=4.5$
$pH=pka+\log \frac{[\text { salt }]}{[\text { acid }]}$
$4.5=4.2+\log \left(\frac{V _s}{V _a}\right)$
$\frac{V _s}{V _a}=2$
$V _s+V _a=300$
$V _a=100 ml$
