Final answer:
The Law of Definite Proportions explains why the product has the same properties regardless of the amounts of H2 and Cl2 used in the reaction. The volume of product HCl formed depends on the initial volumes of H2 and Cl2 and the balanced equation.
Step-by-step explanation:
The result that the product has the same properties regardless of the relative amounts of H2 and Cl2 used is interpreted in terms of the Law of Definite Proportions. This law states that a given chemical compound always contains the same elements in the exact same proportions by mass. In this reaction, hydrogen and chlorine react in a 1:1 stoichiometric ratio, forming the product HCl. The volume of HCl formed when a volume of H2 reacts with an equal volume of Cl2 depends on the initial volumes and the balanced equation.