What are the conjugate acid and base of H_2PO_4?

Aug 25, 2016

Simply add or substract a proton from ${H}_{3} P {O}_{4}$.

Explanation:

Phosphoric acid is the parent acid, i.e. ${H}_{3} P {O}_{4}$.

Remove a proton from this, we get, ${H}_{2} P {O}_{4}^{-}$ as the conjugate base. Or rather phosphoric acid donates a proton to water to give ${H}_{3} {O}^{+}$ and ${H}_{2} P {O}_{4}^{-}$.

And remove a proton from ${H}_{2} P {O}_{4}^{-}$ to get $H P {O}_{4}^{2 -}$ as the conjugate base.

In water that's about as far as you go. But when we speak of conjugate acidity/basicity all we are doing is exchanging protons, ${H}^{+}$ with the solvent. Mass and charge are conserved, as they must be.