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An acid-base indicator which is a weak a...

An acid-base indicator which is a weak acid has a `pK_(In)` value =5.45. At what concentration ratio of sodium acetate to acctic acid would the indicator show a colour half-way between those of its acid and conjugate base forms ?
`[pK_(a)` of acetic acid =4.75, log 2=0.3]

A

`4:1`

B

`6:1`

C

`5:1`

D

`3:1`

Text Solution

Verified by Experts

`pK_(a)=5.45`
`pH=pK_("HIn")+log ((["Base"])/(["Acid form"]))rArr pH=pK_(HIn)=5.45`
For a buffer solution
`pH=pK_(a)+log(([CH_(3)COOH])/([CH_(3)COOH]))rArr 5.45=4.75+log(([CH_(3)COOH])/([CH_(3)COOH]))`
`0.7=log(([CH_(3)COONa])/([CH_(3)COOH]))rArr (5)/(1)=([CH_(3)COONa])/([CH_(3)COOH])`
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