Final answer:
The conjugate base of HPO₄²⁻ is PO₄³⁻, which is formed when HPO₄²⁻ donates a proton, decreasing its charge by one.
Step-by-step explanation:
The formula of the conjugate base of HPO₄²⁻ is PO₄³⁻. When HPO₄²⁻ acts as an acid, it donates a proton (H⁺) according to the Brønsted-Lowry acid-base theory. This donation results in the formation of its conjugate base, PO₄³⁻. The process for deriving the conjugate base is simply the removal of a proton from the original acid, leading to a decrease in its charge by one unit.
Looking at the reaction HPO₄²⁻(aq) + H₂O(l) → H₃O⁺(aq) + PO₄³⁻(aq), it becomes clear that HPO₄²⁻ is the acid since it donates a proton to water, forming hydronium ion (H₃O⁺) and the conjugate base PO₄³⁻.