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

Answer

B

Explanation

`HIn hArr H^(o+) + Ind^(Theta)`,
`pH = pK_(Ind) + "log" ([Ind^(Theta)])/([HIn])`
`pH_(1) = pK_(Ind) + "log" (20)/(80) = pK_(Ind) - 2 log 2`
`pH_(2) = pK_(Ind) +"log" (80)/(20) = pK_(Ind) +2 log2`
`Delta(pH) = pH_(2) - pH_(1) = 4 log 2 = 4 xx 0.3 = 1.2`.