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]

Options

A. `4 : 1`
B. `6 : 1`
C. `5 : 1`
D. `3 : 1`

Answer

C

Explanation

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