The HPO42- ion is amphoteric, meaning that it can act as either an acid or a base. Which of the following is the correct conjugate acid of this molecule?
The correct conjugate acid of the HPO4^2− (hydrogen phosphate) ion is H2PO4^− (dihydrogen phosphate).
Explanation:
Amphoteric substances can act as both acids and bases, depending on the conditions. The HPO4^2− ion, being amphoteric, can either donate a proton (H⁺) to act as an acid or accept a proton to act as a base.
- As an acid:
- When HPO4^2− acts as an acid, it donates a proton (H⁺). This leads to the formation of H2PO4^− (dihydrogen phosphate). The proton loss decreases the negative charge on the molecule from -2 to -1.
- The equation for this reaction is: HPO42−→H2PO4−+H+\text{HPO}_4^{2-} \rightarrow \text{H}_2\text{PO}_4^{-} + \text{H}^+HPO42−→H2PO4−+H+
- Therefore, H2PO4^− is the conjugate acid of HPO4^2− when it behaves as an acid.
- As a base:
- Conversely, HPO4^2− can accept a proton (H⁺) to act as a base, forming the PO4^3− ion (phosphate).
- The reaction would be: HPO42−+H+→PO43−\text{HPO}_4^{2-} + \text{H}^+ \rightarrow \text{PO}_4^{3-}HPO42−+H+→PO43−
- In this case, PO4^3− is the conjugate base of HPO4^2−.
Thus, the conjugate acid of HPO4^2− is H2PO4^−, as it results from the proton donation, which is a key characteristic of the acid behavior of HPO4^2−.
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