Question

Identify the conjugate acid-base pair respectively from the following equilibrium reaction:

\[ \text{HPO}_4^{2-}\text{(aq)} + \text{H}_2\text{O}\text{(l)} \rightleftharpoons \text{PO}_4^{3-}\text{(aq)} + \text{H}_3\text{O}^{+}\text{(aq)} \]

• A. \(\text{H}_3\text{O}^+\) and \(\text{H}_2\text{O}\)
• B. \(\text{H}_2\text{O}\) and \(\text{HPO}_4^{2-}\)
• C. \(\text{PO}_4^{3-}\) and \(\text{H}_3\text{O}^+\)
• D. \(\text{H}_3\text{O}^+\) and \(\text{HPO}_4^{2-}\)

Hint:

To identify a conjugate acid-base pair quickly:
• species losing \(\text{H}^+\) gives its conjugate base,
• species gaining \(\text{H}^+\) gives its conjugate acid. Always remember: conjugate acid-base pairs differ by exactly one proton.

Answer 1

The Correct Option is A

Concept:
According to the Br\o nsted-Lowry concept:
• An acid is a substance that donates a proton \((\text{H}^+)\).
• A base is a substance that accepts a proton \((\text{H}^+)\).
• A conjugate acid-base pair differs by exactly one proton. In the given reaction, we identify:
• the species which loses \(\text{H}^+\) as the acid,
• the species which gains \(\text{H}^+\) as the base.

Step 1: Identify the acid in the reaction.
From the reaction: \[ \text{HPO}_4^{2-} + \text{H}_2\text{O} \rightleftharpoons \text{PO}_4^{3-} + \text{H}_3\text{O}^+ \] We observe that: \[ \text{HPO}_4^{2-} \rightarrow \text{PO}_4^{3-} \] Here, \(\text{HPO}_4^{2-}\) loses one proton \((\text{H}^+)\). So, \(\text{HPO}_4^{2-}\) acts as an acid and \(\text{PO}_4^{3-}\) is its conjugate base.

Step 2: Identify the base in the reaction.
We also observe that: \[ \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ \] Here, \(\text{H}_2\text{O}\) gains one proton \((\text{H}^+)\). So, \(\text{H}_2\text{O}\) acts as a base and \(\text{H}_3\text{O}^+\) is its conjugate acid.

Step 3: Write the conjugate acid-base pairs clearly.
The first conjugate acid-base pair is: \[ \text{HPO}_4^{2-} \text{ and } \text{PO}_4^{3-} \] The second conjugate acid-base pair is: \[ \text{H}_3\text{O}^+ \text{ and } \text{H}_2\text{O} \] Since the question asks for the conjugate acid-base pair respectively, the correct pair is: \[ \text{H}_3\text{O}^+ \text{ and } \text{H}_2\text{O} \] Hence, the correct answer is:
\[ \boxed{(A)\ \text{H}_3\text{O}^+ \text{ and } \text{H}_2\text{O}} \]