The chemical formula for hydrogen phosphate depends on the charge of the ion.

Here are the possibilities:

* Dihydrogen phosphate ion: H₂PO₄⁻

* Hydrogen phosphate ion: HPO₄²⁻

It's important to note that the term "hydrogen phosphate" is often used ambiguously. Therefore, it is important to specify which hydrogen phosphate ion you're referring to in a particular context.