Question

Which of the following is the conjugate acid of $HPO_4^{ - 2}$ ?
\[
  A.{\text{ }}{H_2}PO_4^ - \\
  B.{\text{ }}PO_4^{ - 3} \\
  C.{\text{ }}{H_3}P{O_4} \\
  D.{\text{ }}{H_3}{O^ + } \\
 \]

Answer 1

Hint- In order to deal with this question first we will discuss the compound hydrogen phosphate and then we will proceed further by using the concept as based on the conjugate acid and conjugate base. The conjugate acid base pair differs by one proton. Further we will also see the reaction involving formation of conjugate acid.

Complete answer:
According to the Bronsted-lowery theory, a conjugate acid is a species which is formed by reaction of protons with a base. a conjugate base is a species which is formed when acid loses a proton.
Hydrogen Phosphate
Hydrogen phosphate is a polyatomic ion with the formula $HPO_4^{ - 2}$ . The three oxygen atoms are connected to the phosphorus atom with single covalent bonds. The one hydrogen atom is attached to one of the oxygen atoms with a single covalent bond.
The reaction representing the formation of conjugate acid and conjugate base is as follow:
Acid + Base $ \to $ conjugate base + conjugate acid
Acid donates hydrogen ion and base is the acceptor of proton. The conjugate acid base pair differs by one proton. Conjugate acid is produced when the base is supplied with a hydrogen ion. Whereas, when an acid donates its proton (hydrogen ion) the conjugate base is formed.
Therefore, the base hydrogen phosphate ion reacts with a proton to form a conjugate acid in the following manner:
$HPO_4^{ - 2} + {H^ + } \to {H_2}PO_4^ - $
Hence, the conjugate acid of $HPO_4^{ - 2}$ is ${H_2}PO_4^ - $ .

So, the correct answer is option A.

Note- A conjugate acid contains one more H atom and one more + charge than the base from which it was made. There is one less H atom in a conjugate base, and one non-charge than the acid that formed it. To produce carbonic acid and hydronium ions, let us take the example of bicarbonate ions that react with water. In the pharmaceutical industry, phosphates are used for the production of medicinal products, such as phosphate and adenosine triphosphate, and for the production of toothpastes.