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_(ln)+"log"(20)/(80)=pK_(ln)-2log2`
`pH_(3)=pK_(ln)+2log`
`pH_(2)-pH_(3)=pK_(ln)+2log2-pK_(ln)-2log2)`
`=4log=21.20`