A definite volume of an aqueous `N//20` acetic acid `(pK_(a) = 4.74)` is titrated with a strongs base. It is found that `75` equal-sized drops of `NaOH` added from a burette effect the complete neutralisation. Find the `pH` when an acid solution is neutralised to the extent of `20%, 40%`, and `80%`, respectively.

Options

A. `4.14`
B. `9.86`
C. `5.34`
D. `8.68`

Answer

A

Explanation

`W_(A) (CH_(3)COOH)` is titrated with `S_(B)(NaOH)` and `20%` of the `W_(A)` is neutralised, so `20%` of the heat of `W_(A)//S_(B) (CH_(3)COONa)` is formed and `80%` of the `W_(A)(CH_(3)COOH)` is left. So an acidic buffer if formed.
`:. ["Acid"] = 80, ["Salt"] = 20`
`pH = pK_(a) + log [("Salt")/("Acid")]`
`= 4.7447 + log [("Salt")/("Acid")]`
`= 4.7447 + "log" (1)/(4)`
`= 4.7447 -2 log2 = 4.7447 - 2xx 0.3010 = 4.14`