Law of definite composition

The law of definite proportions states that a chemical compound always has exactly the same proportion of elements by mass.  Joseph Proust, a French chemist,  first observed it in 1797.  For this reason, it is sometimes called  Proust's law.  t is also called the law of constant composition and the law of definite proportions.  It states that all samples of a given chemical compound have the same elemental composition by mass.  For example, oxygen makes up about 8/9 of the mass of any sample of pure water, while hydrogen makes up the remaining 1/9 of the mass.  The law of definite proportions is the basis of stoichiometry, together with the law of multiple proportions.  However, there are some compounds to which this law cannot be applied.  They are sometimes called non-stoichiometric compounds, which are commonly metals.

Related pages
 Valency
 Stoichiometry
 Atomic orbital
 Atomic physics
 Molecular orbital
 Chemical formula
 Quantum mechanics
 Amount of substance
 Mass-energy equivalence

Category:Chemistry