# What is the conjugate acid of HPO4^2-?

The conjugate acid of any species, is that species plus a proton; so it's ${H}_{2} P {O}_{4}^{-}$.
When we add a proton, we must conserve both mass and charge, we add ${H}^{+}$ to $H P {O}_{4}^{2 -}$ so the conjugate acid is simply ${H}_{2} P {O}_{4}^{-}$. What is the conjugate acid of ${H}_{2} P {O}_{4}^{-}$?