Q40P Thioglycolic acid, 聽HSCH2CO2H, ... [FREE SOLUTION]

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Chapter 20: Q40P (page 678)

Thioglycolic acid, ${HSCH}_{2} {CO}_{2} H$ , a substance used in depilatory agents
(hair removers) has $pKa = 3.42$ . What is the percent dissociation of
thioglycolic acid in a buffer solution at $pH = 3.0$ ?

Short Answer

Expert verified

The dissociation percentage of thioglycolicacid is $- 27.55 %$ .

Step by step solution

Finding [A-] by representing HA and A-

Given that $p H = 3$ and $p K_{a} = 3.42$ . On representing and substituting Henderson-Hasselbalch equation,

role="math" localid="1663238528506" $\begin{array}{l} \log \frac{[H A]}{[A^{-}]} = p K_{a} - p H 鈥� o r 鈥� \log \frac{[A^{-}]}{[H A]} = p H - p K_{a} \\ \log \frac{[A^{-}]}{[H A]} = 3 - 3.42 \\ \log \frac{[A^{-}]}{[H A]} = 0.42 \\ \frac{[A^{-}]}{[H A]} = a n t i \log 鈥� o f 鈥� - 0.42 \\ \frac{[A^{-}]}{[H A]} = 0.38 \\ [A^{-}] = 0.38 [H A] - - - (1) \end{array}$

Finding percent dissociation of thioglycollic acid

There is a presence of both ionized thioglycollic acid at equilibrium and also un-ionized forms.

$\begin{matrix} [H A] + [A^{-}] = 100 % - - - (2) \\ [H A] + 0.38 [H A] = 100 % \\ 1.38 [H A] = 100 % \\ [H A] = \frac{100}{1.38} \\ [H A] = 27.55 % \end{matrix}$