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-)? This species does not exist in water. And what is the conjugate base of bisulfate, HSO_4^- (and this one does exist in water)? Can you tell me the conjugate acid of H_3PO_4? Conserve mass, and conserve charge, and these are trivial questions.
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