"A given compound substance is made up of simpler substances, always united at the same mass ratio."
French chemist Joseph Louis Proust observed that in a chemical reaction the relationship between the masses of the participating substances is always constant. THE Proust's Law Or the Law of Defined Proportions says that two or more elements, when combined to form substances, retain definite proportions.
Note: The mass of a water molecule is 18g and is the sum of the atomic masses of hydrogen and oxygen.
H2 - atomic mass = 1 → 2 x 1 = 2g
O - atomic mass = 16 → 1 x 16 = 16g
So 18g of water always has 16g of oxygen and 2g of hydrogen. The water molecule is in the 1: 8 ratio.
The Lavoisier and Proust Laws are called Ponder Laws because they are related to the mass of chemical elements in chemical reactions.