Which law states that a given compound always contains its elements in fixed ratios by mass?

• A. Le Chatelier's principle
• B. Raoult's Law
• C. Law of Definite Proportions
• D. Law of Multiple Proportions

Answer: (C)

The idea being tested is that a specific compound contains its elements in a fixed mass ratio. No matter how the compound is made or how much you have, the masses of the elements combine in the same constant proportion. This is why a compound has a definite composition, such as water always containing hydrogen and oxygen in the same mass ratio. This principle is known as the Law of Definite Proportions.

Le Chatelier's principle is about how systems at equilibrium respond to changes in conditions; Raoult's Law deals with how the vapor pressure of a solution depends on its composition; the Law of Multiple Proportions concerns the ratios formed when two elements produce more than one compound, yielding small whole-number ratios of masses, not the fixed ratio within a single compound.