Final answer:
According to Le Chatelier's principle, the concentration of H₃PO₄ would increase when H₃O+ is added, while the concentration of H₃PO₋₄ would decrease. The concentration of H₃O+ itself would not change significantly, but the pH of the solution would change significantly.
Step-by-step explanation:
A) The concentration of H₃PO₄ increases when H₃O+ is added.
B) The concentration of H₃PO₋₄ increases when H₃O+ is added.
C) The concentration of H₃O+ changes significantly when H₃O+ is added.
D) The pH changes significantly when H₃O+ is added.
According to Le Chatelier's principle, when H₃O+ is added, the equilibrium of the reaction involving H₃PO₄ and H₃PO₋₄ would shift to counteract the change. Since H₃O+ is an acid, it would react with H₃PO₋₄ to form more H₃PO₄, thus increasing its concentration (option A). The addition of H₃O+ would not lead to a significant change in the concentration of H₃O+ itself (option C). The change in concentration of H₃O₄ would affect the pH of the solution, resulting in a significant change in its value (option D).