The law that states a chemical compound always contains the same elements in exactly the same proportions by weight is known as the

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

Answer: (B)

In a pure compound, the elements are always present in fixed, definite proportions by mass. This idea is the Law of Definite Proportions. No matter how the compound is prepared or where it comes from, its mass ratio of the constituent elements stays the same. For example, water has about 11.2% hydrogen and 88.8% oxygen by mass, and sodium chloride has about 39.3% sodium and 60.7% chlorine. If a sample is impure, the observed ratios can vary, but a true compound maintains a constant composition.

This differs from the Law of Conservation of Mass, which says total mass is preserved during a reaction but doesn’t specify that a compound must have a fixed composition. It also differs from the Law of Multiple Proportions, which deals with how different compounds formed from the same two elements can have simple whole-number ratios. The statement describes the fixed, reliable composition of a pure compound, so it aligns with the Law of Definite Proportions.