Question

What will be the conjugate base of $O{H^ - }$ from the following:
(A) ${O_2}$
(B) ${{H}_2}O$
(C) ${O}^ -$
(D) ${H}^-$

Answer 1

Hint :Acid and base are defined through three theories: Arrhenius- defined acid as a substance that liberates hydrogen ions and base liberates hydroxide ions in a solution. Bronsted-Lowry explains an acid as donor of proton and base as proton receiver and Lewis theory states an acid as electron pair acceptor and base as electron pair donor.

Complete Step By Step Answer:
The concept of conjugate acid and base pair comes from Bronsted-Lowry theory.
To understand the concept of let us take an example:
$HA + {H_2}O \rightleftharpoons {H_3}{O^ + } + {A^- }$ where, HA is an acid
Forward reaction: HA donates hydrogen ion (proton) to water, water receives proton thus acts as a base.
Backward reaction: Now ${H_3}{O^ + }$becomes acidic as it now donates protons to form HA and ${A^- }$ now becomes a base which receives protons.
Thus, in the reversible reaction when two acids and bases are present we call them as conjugate acid-base pairs. As in the above reaction HA is acidic and ${A^ - }$ is it’s conjugate base.
And ${A^- }$ as base then as its conjugate acid.
Thus, a conjugate pair differs in either presence of hydrogen or absence of it.
To form a conjugate acid we need to add a proton to it and for the conjugate base remove the proton.
Hence, the conjugate base of $O{H^ - }$ is ${O^ - }$. Option (C).

Note :
Conjugate acids are formed by removal of protons from a compound in the form of hydrogen ions and conjugate bases are formed by adding protons to a compound in the form of a hydrogen atom.