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

The conjugate base of an acid, any acid, is defined as the acid $\text{LESS}$ a proton, ${H}^{+}$. The conjugate acid of a base, any base, is defined as the base $\text{PLUS}$ a proton.
Phosphoric acid, ${H}_{3} P {O}_{4}$, is the parent acid. If it loses a proton, ${H}^{+}$, we conserve both mass and charge, and ${H}_{2} P {O}_{4}^{-}$ results. And what is the conjugate base of this beasty? Again, conserve mass and charge, and $H P {O}_{4}^{2 -}$ 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, $H P {O}_{4}^{2 -}$? This species does not exist in water. And what is the conjugate base of bisulfate, $H S {O}_{4}^{-}$ (and this one does exist in water)? Can you tell me the conjugate acid of ${H}_{3} P {O}_{4}$? Conserve mass, and conserve charge, and these are trivial questions.