What is Proust's Law?
Proust’s Law is now called the Law of Definite Proportions.
It states that a compound always contains the same proportion of elements by mass
The Law is saying that water, for example, is always H₂O. There are no “intermediate'' compounds. In water, the mass of oxygen always has the same ratio to the mass of hydrogen (mass ratio is O:H = 7.94:1).
To tell if compounds containing the same elements are the same, we determine the ratios of their elemental masses.
Compound 1 contains 15.0 g of hydrogen and 120.0 g oxygen. Compound 2 has 2.00 g of hydrogen and 32.0 g oxygen. Are the compounds the same?
Compound 1: Mass ratio of O:H =
Compound 2: Mass ratio of O:H =
The mass ratios are not the same. The compounds are different.