MCQ
$pH$ of a mixture of $1\, M$ benzoic acid $(pK_a = 4.20)$ and $1\,M C_6H_5$ $COONa$ is $4.5$, what is the volume of benzoic acid required to prepare a $300\, ml$ buffer $[log \,2 = 0.3]$? .....$ml$
- A$200$
- B$150$
- ✓$100$
- D$50$
$4.5=4.2+\log \frac{\mathrm{V}}{300-\mathrm{V}}$
$0.3=\log \frac{\mathrm{V}}{300-\mathrm{V}}$
$\log 2=\log \frac{\mathrm{V}}{300-\mathrm{V}}$
$\therefore 2=\frac{\mathrm{V}}{300-\mathrm{V}}$
$\mathrm{V}=200 \therefore$ Volume of the acid $=300-200=$ $100$
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