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.