Which law states that different samples of a pure compound always contain the same elements in the same proportion by mass?

• A. Law of Multiple Proportions
• B. Octet Rule
• C. Law of Definite Composition
• D. Atomic Mass

Answer: (C)

The idea here is that a pure compound has a fixed composition by mass. In every sample of that compound, the elements appear in exactly the same mass ratio because the atoms combine in a specific, fixed way defined by the compound’s formula. This principle is known as the Law of Definite Proportions (also called Proust’s law). It explains why water always has about 11.19% hydrogen and 88.81% oxygen by mass, and why carbon dioxide has about 27.3% carbon and 72.7% oxygen, regardless of how much of the substance you have. This is different from mixtures, where the proportions can vary. The other options describe different ideas: the Law of Multiple Proportions deals with how different compounds can form from the same elements; the Octet Rule relates to electron configurations and stability; and Atomic Mass is a property of atoms, not a law about fixed composition.