An acid-base indicator has `K_(a) = 3.0 xx 10^(-5)`. The acid form of the indicator is red and the basic form is blue. Then:

Options

A. `pH` is `4.04` when indicator is `75%` red.
B. `pH` is `5.00` when indicator is `75%` blue.
C. `pH` is `5.00` when indicator is `75%` red.
D. `pH` is `5.05` when indicator is `75%` blue.

Answer

A, B

Explanation

`pK_(a) =- log (3xx10^(-5)) =- 0.48 +5 = 4.52`
`{:(,H_(Ind)hArr,Ind^(Theta)+,H^(o+),),(,(Red),(Blue),,),((i),75%,25%,,),((ii),25%,75%,,):}`
(i) `pH = 4.52 + log ((25)/(75)) = 4.52 + "log"(1)/(3) = 4.52 - 0.48 = 4.04`
(ii) `pH = 4.52 + log((75)/(25))`
`=4.52 + 0.48 = 5.0`