Answer:
Weak acid - strong base
Explanation:
H₃PO₄ → Phosphoric acid.
This is a weak that has three dissociations in order to give hydronium to the medium and to produce the phosphate anion. The equations are:
H₃PO₄ + H₂O ⇄ H₃O⁺ + H₂PO₄⁻ Ka1
H₂PO₄⁻ + H₂O ⇄ H₃O⁺ + HPO₄⁻² Ka2
HPO₄⁻² + H₂O ⇄ H₃O⁺ + PO₄⁻₃ Ka3
As the H₃PO₄ is a weak acid then the water behaves as a strong base.
If we follow the Brownsted Lory idea, water becomes a strong base cause it receives the H⁺ from water, then the H₃O⁺ becomes the conjugate weak acid.
Anions from the H₃PO₄, diacid phosphate and monoacid phosphate assume the rol of the conjugate strong base, they all recieve proton but this is a special case, because both anions can recieve all release the proton. That's why, they also are amphoteric
