Question

1 M benzoic acid (pKa = 4.20) and 1M C6H5COONa solutions are given separately. What is the volume of benzoic acid required to prepare a 300 ml buffer solution of pH = 4.5 ? [log 2 = 0.3]

• A. 200 ml
• B. 150 ml
• C. 100 ml
• D. 50 ml

Answer 1

Answer: (C)

100 ml

Answer 2

pH = pKa + log $\frac { \left[ \mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { COO } ^ { - } \right] } { \left[ \mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { COOH } \right] }$ $\Rightarrow$ 4.5 = 4.2 + log $\Rightarrow$ = 2

$\therefore$ volume of C6H5COONa required = V2 = 200 ml

volume of C6H5COOH required = V1 = 100 ml.