What is the conjugate acid-base relationship of (H2PO4)- and (HPO4)-?

The conjugate base of an acid, any acid, is defined as the acid less a proton, ##H^+##. The conjugate acid of a base, any base, is defined as the base plus a proton.

Phosphoric acid, ##H_3PO_4##, is the parent acid. If it loses a proton, ##H^+##, we conserve both mass and charge, and ##H_2PO_4^-## results. And what is the conjugate base of this beasty? Again, conserve mass and charge, and ##HPO_4^(2-)## results. You did not conserve mass and charge in your question; I agree that this is all too easy to do.

What is the conjugate base of biphosphate, ##HPO_4^(2-)##? And what is the conjugate base of bisulfate, ##HSO_4^-##? Can you tell me the conjugate acid of ##H_3PO_4##? Conserve mass, and conserve charge, and these are trivial questions.