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The conjugate base of \[H{O^ - }\] is:
(A) \[{O_2}\]
(B) \[{H_2}O\]
(C) \[{O^ - }\]
(D) \[{O^{2 - }}\]

Last updated date: 22nd Mar 2024
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MVSAT 2024
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Hint: The conjugate base is the negatively charged species obtained when an acid dissociates into its ionic form in the aqueous medium. \[H{O^ - }\] can act as an acid as it has a removable proton.

Complete step by step answer:

We know that Bronsted-Lowry theory includes the concept of conjugate acids and conjugate bases.
Let’s know what characterizes the conjugate base.
- We know according to Bronsted-Lowry theory that when acid dissolves in aqueous medium, it gets dissociated into proton and a negatively charged ion. This negatively charged ion is termed as a conjugate base of this acid. So, we can write that,
\[Acid \to {H^ + } + {\text{Conjugate base}}\]
As an example, we can write that when hydrochloric acid dissolves in aqueous media, it dissociates into a proton and chlorine ion, so here chlorine ion is the conjugate base of hydrochloric acid.

Now, we are asked what will be the conjugate base of \[H{O^ - }\].
Look, whenever we are being asked to find a conjugate base of some species, then we should accept that a given species would act as an acid and will give a proton when it gets dissociated into aqueous media. So, we can write that dissociation reaction of \[H{O^ - }\] in water as:
\[H{O^ - } \to {H^ + } + {O^{2 - }}\]

Thus it is clear that we will obtain \[{O^{2 - }}\] as a negative ion when acid \[H{O^ - }\] will get dissociated. So, we can say from the definition of conjugate base that \[{O^{2 - }}\] will be the conjugate base of \[H{O^ - }\].

So, the correct answer for this question is option (D) \[{O^{2 - }}\].


Do not get confused between conjugate acid and conjugate base. Conjugate acid is a species which is formed when a base accepts a proton. Do not think that as \[H{O^ - }\] is a negatively charged species, it cannot act as an acid.

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