Final answer:
Upon adding a strong acid like HCl to Na2HPO4, the solution will form the weak acid NaH2PO4 and the salt NaCl, potentially reducing the solution's pH. Na2HPO4 can act as both an acid and base, showing amphoteric behavior.
Step-by-step explanation:
When sufficient strong acid is added to a solution containing Na2HPO4, it reacts to form NaH2PO4 and NaCl, where NaH2PO4 is a weak acid and NaCl is a salt. As Na2HPO4 is a weak base and HCl is a strong acid, the resulting weak acid formed suggests that the solution will become more acidic. However, HPO42- anion is amphiprotic, capable of reacting with added H+ to form H2PO4-, and with OH- to form HPO42- and water, indicating buffer-like behavior. Since HPO42- can act as both an acid and a base, it is considered amphoteric.