# What is the Law of Definite Proportions?

The $\text{law of definite proportions}$ or the $\text{law of constant composition}$ holds that a chemical compound always contains its component elements in a fixed ratio, regardless of the source of the compound. Joseph Proust proposed a similar $\text{law of constant composition.}$
An example of this law is that water ALWAYS has a composition by mass of $\frac{1}{8}$ hydrogen, and $\frac{7}{8}$ oxygen. This law allows us to propose empirical formulae, and by extension molecular formulae, if the molecular mass is known.