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 [tex]K_{a}[/tex].
Dissociation of the given compounds or species will be as follows.
[tex]H_{2}CO_{3} \rightleftharpoons H^{+} + HCO^{-}_{3}[/tex]
[tex]H_{2}PO^{-}_{4} \rightleftharpoons H^{+} + HPO^{-}_{4}[/tex]
[tex]H_{3}PO_{4} \rightleftharpoons H_{+} + H_{2}PO^{-}_{4}[/tex]
As, chemical formula of bicarbonate is [tex]HCO^{-}_{3}[/tex] 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 [tex]K_{a}[/tex].
Thus, we can conclude that out of the given species bicarbonate has the lowest [tex]K_{a}[/tex].
