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An acid-base indicator (PK(a) =4.5271) ...

An acid-base indicator (`PK_(a) =4.5271`) has the acid form red and basic form blue. If we need 75% red to be converted into 75% blue form in solution, the change in pH of solution should be:

A

`4.05`

B

`5.0`

C

`0.95`

D

`0.80`

Text Solution

Verified by Experts

The correct Answer is:
C
For `75%` red from `{:(HI _(n) hArr H ^(+) + I _(n) ^(-)) ,( (Red) " "("Blue")), (75 " "25):}`
`K _(a) = ([ H ^(+)] [I _(n) ^(-)])/( [HI _(n )]), pH = pK _(a) + log "" (25)/(75) = 4.5271- 0.4771 = 4. 05`
For `75%` blue form `pH = 4.5271 + log "" (75)/(25) = 4.5271 + 0. 4771 = 5.0`
`Delta pH = 5. 0 - 4. 05 = 0. 95`
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