What is the formula for hydrogen phosphate?

The formula for hydrogen phosphate is represented as HPO4^2-. This ion contains one hydrogen atom bonded to the phosphate ion, which has a chemical formula of PO4^3-. The presence of one hydrogen atom reduces the negative charge of the phosphate ion from -3 to -2, resulting in HPO4^2-.

In contrast, the other options represent different species. PO4^3- is the formula for the phosphate ion itself, which contains no hydrogen atoms and carries a -3 charge. H2PO4- represents dihydrogen phosphate, which has two hydrogen atoms and a -1 charge. Lastly, NH4+ is the ammonium ion and is not related to phosphate at all, as it does not contain phosphorus. Thus, HPO4^2- is the correct representation of hydrogen phosphate, reflecting its composition and charge accurately.