Question

Give reason for the following fact:

H₃PO₄ is a diprotic acid.

Answer 1

1. H₃PO₄ (orthophosphoric acid) has three hydrogen atoms bonded to oxygen.
2. The first hydrogen ion (H⁺) dissociates easily because it is attached to a strongly electronegative oxygen.
3. The second hydrogen ion also dissociates but with more difficulty than the first.
4. The third hydrogen ion is much less acidic because it is bonded directly to phosphorus.
5. Therefore, only two hydrogen ions ionise easily in water, making H₃PO₄ a diprotic acid.

Thus, the acidic behaviour corresponds mainly to two protons released, not all three. This is why H₃PO₄ is called diprotic, even though it has three acidic hydrogens by formula.