For a polybasic acid, the dissociation constants have a different values for each step, e.g.,
$
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Question

For a polybasic acid, the dissociation constants have a different values for each step, e.g.,
$
\begin{aligned}
& \mathrm{H}_3 A ightleftharpoons \mathrm{H}^{+}+\mathrm{H}_2 A^{-} ; K=K_{a_1} \
& \mathrm{H}_2 A^{-} ightleftharpoons \mathrm{H}^{+}+\mathrm{H} A^{2-} ; K=K_{a_2} \
& \mathrm{H} A^{2-} ightleftharpoons \mathrm{H}^{+}+A^{3-} ; K=K_{a_3}
\end{aligned}
$
What is the observed trend of dissociation constants in successive stages?

Professor PC Thomas Classes · Answer

(a) : Higher order ionisation constant is smaller than the lower order ionisation constant in case of polyprotic acid. Thus, $K_{a_1}>K_{a_2}>K_{a_3}$