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_2X \rightleftharpoons H^+ + HX^- (K_{a1} ) $
$ HX^- \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} $