The law stating that a chemical compound has fixed, definite mass ratios of elements is known as

• A. Law of Conservation of Mass
• B. Law of Definite Proportions
• C. Law of Multiple Proportions
• D. Atomic Mass Unit

Answer: (B)

The main idea being tested is that a chemical compound has a fixed, definite ratio of masses of its elements. This means that no matter how much of the compound you have or where it comes from, the elements combine in the same proportion by mass. This consistent composition is called the Law of Definite Proportions (Proust’s Law). For example, water always contains hydrogen and oxygen in the same mass ratio (about 1 part hydrogen to 8 parts oxygen). This concept is what makes a substance have a specific formula and composition. The other ideas refer to different principles: conservation of mass concerns total mass in reactions, the law of multiple proportions deals with different compounds formed from the same elements, and the atomic mass unit is just a unit of mass, not a law about composition.