The ionisation constant of an acid base indicator (a weak acid) is `1.0 xx 10^(-6)`. The ionised form of the indicator is red and unionised form is blue. The p`H` change required to alter the colour of indicator form `80%` red is

Options

A. `1.40`
B. `1.20`
C. `0.80`
D. `2.00`

Answer

B

Explanation

`Hln hArr H^(+) + ln^(-)`
`pH = pK_(In) + "log" ([In^(-)])/(["Hin"])`
`pH_(1) = pK_(In) + "log" (20)/(80) = pK_(In) - 2log2`
`pH_(2) - pH_(1) = pK_(In) + 2log2 - (pK_(In) - 2log2)`
`= 4 log 2 = 1.20`