A definite volume of a N/20 `CH _(3) COOH (pK _(a) =4.7447)` is titrated with a strong base (NaOH). It is found that 80 equal sized drops of NaOH, added from a burette effects the complete neutralisation. Find the pH, when the acid solution is neutralised to the extent of 20%

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 salt of `W_(A)//S_(B)(CH_(3)COONa)` is formed acid and 80% of the `W_(A)(CH_(3)COOH)` is left. So an acidic buffer is formed.
`therefore ["Acid"] = 80, ["Salt"] = 20`
`pH = pKa + log[("Salt")/("Acid")] = 4.7447 + log [("Salt")/("Acid")] = 4.7447 + log""(1)/(4)`
`= 4.7447 - 2 log 2 = 4.7447 - 2 xx 0.3010 = 4.14`