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An acid-base indicator has a K(a) = 3.0 ...

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

A

pH is 4.05 when indicator is 75% red.

B

pH is 5.00 when indicator is 75% blue.

C

Both (a) and (b) are correct.

D

None of these.

Text Solution

Verified by Experts

The correct Answer is:
C
`underset("Red")("HIn") overset(K_("Ind"))hArr H^(o+) + underset("Blue")("Ind")^(ɵ)`
Use : `pH = pK_("Ind") + log"" (["Ind"^(ɵ)])/(["HIn"])`,
`pK_("Ind") = -log(3 xx 10^(-5)) = 4.52`
75% Red `rArr (["HIn"])/(["Ind"^(ɵ)]) = (3)/(1)`
`rArr pH = pK_("Ind") + log""(1)/(3) = 4.05`
75% Blue `rArr (["HIn"])/(["Ind"^(ɵ)]) = (1)/(3)`
`rArr` pH = `pK_("Ind") + log""(3)/(1) = 5.00`
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