Question

Which of the following has the lowest Ka?a. carbonic acid b. dihydrogen phosphate c. phosphoric acid d. bicarbonate

Answer 1

Explanation:

A compound or molecule which will have least dissociation and that is not able to given hydrogen ion easily upon dissociation will also have a low value of $K_{a}$.

Dissociation of the given compounds or species will be as follows.

 $H_{2}CO_{3} \rightleftharpoons H^{+} + HCO^{-}_{3}$

 $H_{2}PO^{-}_{4} \rightleftharpoons H^{+} + HPO^{-}_{4}$

 $H_{3}PO_{4} \rightleftharpoons H_{+} + H_{2}PO^{-}_{4}$

As, chemical formula of bicarbonate is $HCO^{-}_{3}$ and due to the presence of a negative charge it is difficult to lose a positively charged hydrogen ion. This is because oppositely charged ions will be bonded by strong force of attraction.

Hence, it will not easily lose a hydrogen ion due to which bicarbonate has the lowest $K_{a}$.

Thus, we can conclude that out of the given species bicarbonate has the lowest $K_{a}$.