Question

Which reactants in the reactions below
are acting as Br©ªnsted-Lowry bases?
NH4
+(aq) + OH−(aq) <-> NH3(aq) + H2O(l)
H2PO4
−(aq) + H2O(l) <-> H3O+(aq) + HPO4
2−(aq)

is the answer H2O
and the H3O+

Answer 1

Both H2O and H3O^+ are products, not reactants. The question asks for the acid/base in the reactants. For whatever it is worth, H2O in equation 1 is acting as an acid because it is donating a H to NH3 to form NH4^+. In equation 2, H3O^+ is acting as an acid by donating a H to HPO4^-2. None of that answers your question but it may clarify it. What you want to do is to pick either NH4^+ or OH^- in equation 1 and determine which is the acid and which is the base. In equation 2 the choice is between H2PO4^- and H2O.