The conjugate acid of `HPO_(4)^(2-)` is

Options

A. `PO_(4)^(3-)`
B. `H_(2)PO_(4)^(-)`
C. `H_(3)PO_(4)`
D. `HPO_(3)`

Explanation

To find the conjugate acid of \( HPO_4^{2-} \), we need to understand the concept of conjugate acids and bases.

### Step-by-Step Solution:

1. **Identify the Species**: The species in question is \( HPO_4^{2-} \).

2. **Understand Conjugate Acids**: A conjugate acid is formed when a base accepts a proton (H⁺ ion). Conversely, when an acid donates a proton, it forms its conjugate base.

3. **Determine the Proton Addition**: Since \( HPO_4^{2-} \) is a base, we can find its conjugate acid by adding a proton (H⁺) to it.

4. **Perform the Addition**:
\[ HPO_4^{2-} + H^+ \rightarrow H_2PO_4^{-} \]
Here, \( HPO_4^{2-} \) accepts a proton to become \( H_2PO_4^{-} \).

5. **Conclusion**: The conjugate acid of \( HPO_4^{2-} \) is \( H_2PO_4^{-} \).

### Final Answer:
The conjugate acid of \( HPO_4^{2-} \) is \( H_2PO_4^{-} \).