Which law states that compounds are made up of fixed proportions?

• A. Law of Definite Proportions
• B. Law of Conservation of Mass
• C. Dalton's Law
• D. Henry's Law

Answer: (A)

Compounds have fixed, definite mass ratios of their elements. No matter where a compound comes from or how it’s made, its mass composition stays the same. For example, water always contains hydrogen and oxygen in a constant ratio (two hydrogen atoms for every one oxygen atom), and sodium chloride has a specific proportion of sodium to chlorine by mass. This consistent, fixed composition is exactly what the law describes. The other laws address different ideas: mass conservation in reactions, how gas pressures add up in mixtures, or how gas solubility relates to pressure, none of which specify that a compound has fixed proportions.