What principle describes that a chemical compound contains elements in a fixed ratio by mass?

The principle that describes a chemical compound containing elements in a fixed ratio by mass is known as the Law of Constant Composition. This law asserts that a chemical compound is composed of the same proportion of elements by mass regardless of the sample size or source of the compound. For example, water (H₂O) is always composed of two hydrogen atoms and one oxygen atom, which corresponds to a specific mass ratio. This constancy is fundamental in chemistry and allows for consistent identification and formulation of substances.

Understanding this concept is crucial for studying chemical reactions and stoichiometry, as it lays the foundation for calculating the proportions of elements involved in different chemical processes.