HC2H3O2 + H2O <===> H3O+ + C2H3O2¯

HC2H3O2 and C2H3O2¯ is one conjugate pair.

H2O and H3O+ is the other.

NH3 + H2O <===> NH4+ + OH¯

NH3 and NH4+ is one pair.

H2O and OH¯ is the other.

Notice that H2O in the first equation is acting as a base and in the second equation is acting as an acid. Return to Brønsted-Lowry Theory