What acid is HPO4?

HPO₄^2- is not an acid itself, but rather the conjugate base of an acid. Specifically, it is the conjugate base of dihydrogen phosphate (H₂PO₄^-). Dihydrogen phosphate, in turn, is formed from phosphoric acid (H₃PO₄). Therefore, HPO₄^2- relates back to phosphoric acid.

Here's a breakdown of the phosphoric acid and its related ions through successive deprotonations (loss of H⁺):

1. Phosphoric Acid (H₃PO₄): This is the triprotic acid.

2. Dihydrogen Phosphate (H₂PO₄^-): Formed when phosphoric acid loses one proton (H⁺).

3. Hydrogen Phosphate (HPO₄^2-): Formed when dihydrogen phosphate loses another proton. This is the species the question is asking about.

4. Phosphate (PO₄^3-): Formed when hydrogen phosphate loses its final proton.

Therefore, while HPO₄^2- isn't an acid, it's directly related to and derived from phosphoric acid (H₃PO₄). It exists as an ion in solution, depending on the pH.