Which of the following is the correct formula for the hydrogen phosphate ion?

• A. H_2PO_4^{-}
• B. HSO_4^{-}
• C. HPO_4^{2-}
• D. PO_4^{3-}

Answer: (C)

Verified step by step guidance

1. Understand that the problem is asking for the correct chemical formula of the hydrogen phosphate ion, which is a polyatomic ion derived from phosphoric acid (H₃PO₄) by losing hydrogen ions (protons).
2. Recall the series of phosphate-related ions formed by stepwise deprotonation of phosphoric acid: H₃PO₄ (phosphoric acid), H₂PO₄⁻ (dihydrogen phosphate), HPO₄²⁻ (hydrogen phosphate), and PO₄³⁻ (phosphate).
3. Identify that the hydrogen phosphate ion specifically has lost two protons compared to phosphoric acid, resulting in the formula HPO₄²⁻, where the charge is 2- due to the remaining negative charges on the oxygen atoms.
4. Compare the given options to the known formulas: H₂PO₄⁻ is dihydrogen phosphate, HSO₄⁻ is hydrogen sulfate (not related to phosphate), HPO₄²⁻ is hydrogen phosphate, and PO₄³⁻ is phosphate ion.
5. Conclude that the correct formula for the hydrogen phosphate ion is \(HPO_4^{2-}\) based on the stepwise deprotonation and charge balance.