An acid-base indicator, HIn, dissociates according to the following reaction in

Question

An acid-base indicator, HIn, dissociates according to the following reaction in an aqueous solution.

HIn(aq) --> In-aq) + H+ (aq)

The protonated form of the indicator, HIn, has a molar absorptivity of 3057 M–1·cm–1 and the deprotonated form, In–, has a molar absorptivity of 20940 M–1·cm–1 at 440 nm. The pH of a solution containing a mixture of HIn and In– is adjusted to 5.92. The total concentration of HIn and In– is 0.000190 M. The absorbance of this solution was measured at 440 nm in a 1.00 cm cuvette and was determined to be 0.833. Calculate pKa for HIn.

Explanation / Answer

ACCORDING TO HENDERSON EQUATION,WE CAN CALCULATE PKa value.

PH=Pka +log ( In-)   / ( HIn) ; molar absorptivity is intensive property so the given data is constant value.

5.92 = PKa + log( 1   /.000190 )

         =Pka +log (5263 )

        =pka + 3.7212

Pka     for   HIn is     5.92   -3.7212   =2.1988.

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