To which of the solution, addition of water would not effect the `pH` ? .

Options

A. `100 mL of 0.2 M CH_(3) COOH + 100mL of 0.1 M NaOH`
B. `100 mL of 0.2 M CH_(3) COOH + 100mL of 0.2 M NaOH`
C. `200 mL of 0.2 M CH_(3) COOH + 100mL of 0.1 M NaOH`
D. `100 mL of 0.2 M CH_(3) COOH + 200mL of 0.1 M NaOH`

Answer

A, C

Explanation

Addition of `H_(2)O` will not change `pH` only for buffer solution. (Buffer is formed if `W_(A)` or `W_(B)` is left after neutralisation)
a. Acid buffer: `CH_(3)COOh = 100 xx 0.2 = 20 "mmol" NaOH = 100 xx 0.1 = 10` mmol
`10 "mmoles"` of `CH_(3)COOH` will react with `10 "mmol"` of `NaOH` to form `10 "mmol"` of `CH_(3)COONa` and `10 "mmol"` of `CH_(3)COOH` is left.
b. mmol of `CH_(3)COOH = 100 xx 0.2 = 20`
mmol of `NaOH = 100 xx 0.2 = 20`
(Salt of `W_(A)//S_(B)` is formed)
c. mmol of `CH_(3)COOH = 200 xx 0.2 = 40`
mmol of `NaOH = 100 xx 0.1 = 10`
mmol of `CH_(3)COOH` left `= 40 - 10 = 30`
mmol of `CH_(3)COONa` formed `= 10`.
(Buffer is Formed)
d. mmol of `CH_(3)COOH = 100 xx 0.2 = 20`
mmol of `NaOH = 200 xx 0.2 = 40`
mmol of NaOH left `= 40 - 20 = 20 S_(B)` is left.