Question

Which of the following does not act as Bronsted acid?

• A. ${NH}_4^{+}$
• B. ${CH}_{3}CO{O}^{-}$
• C. $H{CO}_3^{-}$
• D. $H{SO}_3^{-}$

Answer 1

Answer: (B)

${CH}_{3}CO{O}^{-}$

The concept of conjugate Acid-Base pair was given by Lowry-Bronsted Theory.
According to the theory:
Acids are proton $\left(H^{+}\right)$ donors.
Bases are proton $\left(H^{+}\right)$ acceptors.

Hence, in order to get the conjugate base of a species, we should consider substance as an acid and vice versa.
For example, $H_{2}S{O}_4\to HS{O}_4^{-}+H^{+}$
Therefore, conjugate base of $H_{2}S{O}_4$ is $HS{O}_4^{-}$.

$O{H}^{-}\to O^{-2}+H^{+}$
Hence, conjugate base of $O{H}^{-}$ is $O^{-2}$.

A substance can act as a Bronsted Acid if it can donate a proton. Out of all the given options, $C{H}_{3}CO{O}^{-}$ cannot donate a proton. Hence, it cannot act as a Bronsted Acid.