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Amino Acids

A certain mon...

Question

A certain monoprotic acid (weak) serves as indicator. Assuming that colour change is seen when 1/3rd of the indicator has been converted to ions and that at end point the pH of solution is $6$ , What is the value of $p K_{l n}$ ?

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Solution

Equilibrium reaction is
$H I n ⇌ H^{+} + {I n}^{-}$
$p H = p K_{l n} + {log}_{10} \frac{[{I n}^{-}]}{[H I n]}$
$6 = p K_{l n} + {log}_{10} \frac{[0.33]}{[0.67]}$ (1/3rd conversion)
$p K_{l n} = 6.307$

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