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The conjugate acid of H2O is H3O+
The conjugate acid of any substance is given by removing an acidic hydrogen. In the case of ammonium ion, the conjugate base is ammonia.
The conjugate base of H2PO3- ion is HPO3-2 ion.
H3PO4 (aq) + H2O (l) ---> 2H3O+ (aq) + PO4-3 (aq)
donor acid + acceptor base ---> conjugate acid + conjugate base
the answer above is wrong
to form a conjugate, the ion H2PO4 - must lose a hydrogen ion H+
i.e
H2PO4 - -H+ = HPO4 2-
(conjugate base)
HSO4