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Question: The conjugate acid of \({\text{HP}{{\text{O}}_{\text{4}}}^{2-}}\) is: A. \({{{\text{H}}_{\text{3}}...

The conjugate acid of HPO42−{\text{HP}{{\text{O}}_{\text{4}}}^{2-}} is:
A. H3PO3{{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{3}}}}
B. H3PO4{{{\text{H}}_{\text{3}}}\text{P}{{\text{O}}_{\text{4}}}}
C. H2PO4−{{{\text{H}}_{\text{2}}}\text{P}{{\text{O}}_{\text{4}}}^{-}}
D. PO43−{\text{P}{{\text{O}}_{4}}^{3-}}

Explanation

Solution

Hint: In order to answer this question, we need to know the structure of the acids that are mentioned in the options. The structure of the acid that is mentioned in the question should also be known.

Step by step answer:
We know that the conjugate base of an acid, any acid is defined as the acid less of proton, H+{{{\text{H}}^{\text{+}}}}
The conjugate acid of a base, any base is defined as the base plus a proton.
The above mentioned statement can be proved by the fact that, according to Bronsted Lowry theory, a conjugate acid is a species which is formed by reaction of protons with the base. And a conjugate base is a species which is formed with acid and loses a proton.
The reaction representing the formation of conjugate acid and conjugate base is as follows:
Acid + Base = Conjugate Acid + Conjugate Base\text{Acid + Base = Conjugate Acid + Conjugate Base}

Acid donates hydrogen ions and the base is an acceptor of protons. The conjugate acid base pair diffuses by one proton. Conjugate acid is formed when a hydrogen ion is added to the base whereas the conjugate base is formed when an acid donates its proton that is hydrogen ion.

Therefore, the base hydrogen phosphate ion reacts with a proton to form a conjugate acid.
The reaction of hydrogen phosphate ion with a proton to form its conjugate acid is given below:
HPO42−+ H+ → H2PO4−\text{HP}{{\text{O}}_{4}}^{2-}+\text{ }{{\text{H}}^{\text{+}}}\text{ }\xrightarrow{{}}\text{ }{{\text{H}}_{\text{2}}}\text{P}{{\text{O}}_{\text{4}}}^{-}
Therefore, the conjugate acid that is formed is H2PO4−{{{\text{H}}_{\text{2}}}\text{P}{{\text{O}}_{\text{4}}}^{-}}, which is dihydrogen phosphate ion.

So the correct answer is Option C.

Note: We should know that the conjugate acid of a weak base is a strong acid. And the conjugate acid of a strong base is a weak acid. The conjugate base of an acid is the anion that results when the acid molecule loses its hydrogen to a base.