Final answer:
H₃PO₄, or phosphoric acid, is a weak acid. It is a polyprotic acid with multiple ionizable protons, and its conjugate base can exhibit amphoteric behavior, acting as both an acid and a base in different contexts.
Step-by-step explanation:
H₃PO₄, known as phosphoric acid, is a polyprotic acid, which means it contains more than one ionizable proton and can donate these protons in a stepwise manner. The fully protonated species is typically the strongest acid compared to its deprotonated forms. In the case of phosphoric acid, it is easier to remove the first proton from the neutral molecule, thus, after losing one proton, the acid strength decreases and the pKa increases. This concept also applies to other polyprotic acids like H₂SO₄ (sulfuric acid) and H₂CO₃ (carbonic acid).
Additionally, phosphoric acid's conjugate bases, such as HPO₄²⁻, can act as bases in reactions with water, donating a proton and ultimately forming OH⁻ (hydroxide ion), showcasing amphoteric behavior.
Considering the reaction HCl + Na₂HPO₄ → NaH₂PO₄ + NaCl, HCl is a strong acid and Na₂HPO₄ acts as a weak base, indicating that H₂PO₄⁻ is a weak acid. Thus, H₃PO₄ fits the description of a weak acid.