Which law states that when two elements form more than one compound, the ratio of the masses of one element that combines with a fixed mass of the other is in small whole-number ratios?

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

Answer: (C)

Two elements forming more than one compound follow a pattern: the masses of one element that combine with a fixed amount of the other appear in simple, small whole-number ratios. This is the Law of Multiple Proportions. A classic illustration is carbon and oxygen: in CO and CO2, fixing the amount of carbon shows the oxygen needed as 1 part for CO and 2 parts for CO2, a 1:2 ratio. The same idea holds for nitrogen and oxygen with NO and NO2, where the oxygen mass relative to a fixed nitrogen mass is a simple whole-number ratio. This demonstrates that when you vary the compound formed from the same two elements, the combinations occur in discrete, small whole-number relationships. The other statements describe different ideas: mass is conserved in reactions, a given compound has a fixed elemental ratio, and atomic number is a property of elements, not a law about how elements combine into compounds.