H2 PO4

If you mean dihydrogen phosphate, the species is H2PO4− (dihydrogen phosphate ion).

Key points:

• Formula and charge: H2PO4−; derived from phosphoric acid H3PO4.
• Structure: a phosphate tetrahedron with one P=O, two P–OH groups, and one P–O−; the negative charge is delocalized over the oxygens.
• Acidity/basicity (pKa values):
  • pKa1 ≈ 2.15 for H3PO4 → H2PO4−
  • pKa2 ≈ 7.20 for H2PO4− → HPO4^2−
  • pKa3 ≈ 12.35 for HPO4^2− → PO4^3−
  • H2PO4− is amphiprotic: it can donate a proton to form HPO4^2− or accept a proton to form H3PO4.
• Buffering: H2PO4−/HPO4^2− is a common phosphate buffer around pH ~ 7.2. For a buffer, use pH ≈ pKa2 + log([HPO4^2−]/[H2PO4−]).
• Reactions:
  • With a strong base: H2PO4− + OH− → HPO4^2− + H2O
  • With a strong acid: H2PO4− + H+ → H3PO4
• Salts and uses: Sodium dihydrogen phosphate (NaH2PO4) is a common salt; dihydrogen phosphate salts are used in buffers and food/pharma contexts.

If you meant something else (like H3PO4, HPO4^2−, or the neutral H2PO4 group in a compound), tell me and I’ll tailor the notes. Want a quick worked example (e.g., calculating pH of a phosphate buffer with given concentrations)?