Final answer:
When 1.5 equivalents of H+ are added to a phosphate solution, HPO4^2- is the predominant species due to the nature of polyprotic acids like phosphate undergoing sequential deprotonation.
Step-by-step explanation:
When 1.5 equivalents of H+ has been added to a solution of phosphate ion, the species that predominates in the solution is HPO42-. This is because phosphate (PO43-) is a polyprotic acid that can sequentially lose three protons. After one equivalent of H+ is added, HPO42- becomes the dominant species due to the first deprotonation.
When 1.5 equivalents of acid are added, there would be a mixture, but HPO42- would still be the predominant species because the next deprotonation step, which produces H2PO4-, is less favorable due to a significantly smaller ionization constant.