Question

What are the conjugate acid and base of `H_2PO_4`?

Answer 1

Simply add or substract a proton from `H_3PO_4`.

Explanation:

Phosphoric acid is the parent acid, i.e. `H_3PO_4`.

Remove a proton from this, we get, `H_2PO_4^-` as the conjugate base. Or rather phosphoric acid donates a proton to water to give `H_3O^+` and `H_2PO_4^-`.

And remove a proton from `H_2PO_4^-` to get `HPO_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.