A. H3PO4 is the acid and its conjugate base is H2PO4- H2O is the base and its conjugate acid is H3O+. B. H2PO4- is the acid and its conjugate base is H2PO4^2- H2O is the base and its conjugate acid is H3O+. Solution A. H3PO4 is the acid and its conjugate base is H2PO4- H2O is the base and its conjugate acid is H3O+. B. H2PO4- is the acid and its conjugate base is H2PO4^2- H2O is the base and its conjugate acid is H3O+..