What is the Law of Definite Proportions?

The `"law of definite proportions"` o il `"law of constant composition"` holds that a chemical compound always contains its component elementi in a fixed ratio, regardless of the source of the compound. Joseph Proust proposed a similar `"law of constant composition."`

An example of this law is that water ALWAYS has a composition by mass of `1/8` hydrogen, and `7/8` oxygen. This law allows us to propose empirical formulae, and by extension molecular formulae, if the molecular mass is known.