A certain mixture of `HCl` and `CH_(3)-COOH` is `0.1 M` in each of the acids. `20 ml` of this solution is titrated against `0.1 M NaOH`. By how many units does the `pH` change from the start to the stage when the `HCl` is almost completely neutralised and acidic acid remains unreacted? `K_(a)` for acetic acid `=2xx10^(-5)`.

Options

A. `1.5`
B. `3`
C. `2`
D. `3.25`

Answer

B

Explanation

mmoles of `HCl=0.1xx20=2`
mmoles of `CH_(3)COOH=0.1xx20=2`
After titration of `HCl` by `NaOH`
`[CH_(3)COOH]=(2)/(40)=(1)/(20)M`
`pH=(1)/(2)(pK_(a)-log C)`
`=(1)/(2)[5-log2-log``((1)/(20))]=3`