The conjugate acid of HPO4²⁻ (hydrogen phosphate) is H2PO4⁻ (dihydrogen phosphate).
Explanation: When HPO4²⁻ acts as a base, it can accept a proton (H⁺). This addition of a proton transforms it into its conjugate acid, H2PO4⁻. In essence, the conjugate acid is formed by the base gaining a proton, which reflects the Brønsted-Lowry theory of acids and bases. In solution, this transformation plays a crucial role in buffering systems and impacts various biochemical processes.