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Question: When a solution of benzoic acid was titrated with NaOH the pH of the solution when half the acid neu...

When a solution of benzoic acid was titrated with NaOH the pH of the solution when half the acid neutralized was 4.2. Dissociation constant of the acid is

A

6.31×10−56.31 \times 10^{- 5}

B

3.2×10−53.2 \times 10^{- 5}

C

8.7×10−88.7 \times 10^{- 8}

D

6.42×10−46.42 \times 10^{- 4}

Answer

6.31×10−56.31 \times 10^{- 5}

Explanation

Solution

C6H5COOH+NaOHC6H5COONa+H2O\mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { COOH } + \mathrm { NaOH } \quad \mathrm { C } _ { 6 } \mathrm { H } _ { 5 } \mathrm { COONa } + \mathrm { H } _ { 2 } \mathrm { O }

Final 0.5 0.5

It is a buffer solution of weak acid and its salt

pH=pHa+log⁡[salt][acid]pH = pH_{a} + \log\frac{\lbrack salt\rbrack}{\lbrack acid\rbrack}

pka=4.2pk_{a} = 4.2

ka=6.31×10−5k_{a} = 6.31 \times 10^{- 5}