`18mL` of mixture of `CH_(3)COOH` and `CH_(3)COONa` required `6mL` of `0.1M NaOH` for neutralisation of the acid `12mL` of `0.1M HC1` for reaction with salt, separately. If `pK_(a)` of the acid is `4.75`, what is the `pH` of the mixture

Options

A. `4.5`
B. `4.6`
C. `4.75`
D. `5.05`

Answer

D

Explanation

For neutralisation of `CH_(3)COOH` with `NaOH`
mmol of `CH_(3)COOH` = mmol of `NaOH`
`= 6 xx 0.1 = 0.6`
For neutralisation `CH_(3)COONa` with `HC1`
mmol of `CH_(3)COONa` mmol of `HC1`
`12 xx 0.1 = 1.2`
Concentration of acid and slat in `18mL` of mixture
`[CH_(3)COOH] = (0.6 "mmol")/(18mL)`
`[CH_(3)COONa] = (1.2 "mmol")/(18mL)`
`pH = 4.75 + log ((1.2)/(0.6))`
`= 4.75 + log 2 = 4.75 + 0.3 = 5.05`