Question

Which one of the following is true for any diprotic acid, $H_{2}X$ ?

• A. $K_{a2}=K_{a1}$
• B. $K_{a2}>K_{a1}$
• C. $K_{a2}<K_{a1}$
• D. $K_{a2}=\frac{1}{K_{a1}}$

Answer 1

Answer: (C)

$H_{2}X\rightleftharpoons H^{+}+H{X}^{-}(K_{a1})$
$H{X}^{-}\rightleftharpoons H^{+}+X^{2-}(K_{a2})$
In 1st equation hydrogen ion is formed from neutral molecule whereas in IInd equation it comes from negatively charged species. Due to negative charge removal of proton is difficult.
So, $K_{a1}>K_{a2}$