Final answer:
The correct completion for the Brønsted-Lowry reaction is HPO42- + H2O, with PO43- being the base accepting a proton from the acid H3O+, resulting in HPO42- and H2O.
Step-by-step explanation:
The correct completion for the Brønsted-Lowry acid-base reaction PO43- + H3O+ <---> is HPO42- + H2O. In this reaction, the phosphate ion (PO43-) acts as a Brønsted-Lowry base because it accepts a proton (H+) from the hydronium ion (H3O+), which is a Brønsted-Lowry acid, therefore, donating the proton. Consequently, the products are hydrogen phosphate (HPO42-) and water (H2O), forming a conjugate acid-base pair.
In reference to the exercises provided, the essence of Brønsted-Lowry theory is that acids donate protons and bases accept protons. For example, H2PO4- can either donate a proton to act as an acid or accept a proton to act as a base. This flexibility demonstrates the underlying principle of the dual nature of substances under the Brønsted-Lowry acid-base definition.