Law of Definite Proportion, also called Proust's Law or Law of Constant Composition, defines that the elements that make up a chemical compound are usually arranged in a specified mass ratio regardless of the source or preparation. The law of definite proportion can also be expressed in another way.
How does the Law of Definite Proportions work?
Using the law of definite proportions, the composition of compounds will always be the same by mass. In Chemistry, stoichiometry is based on this law.
Statement: Chemical compounds consist of elements that are always present at fixed ratios (in terms of their mass) according to the law of definite proportions as well as the law of constant proportions. In this ratio, neither the source nor the method of preparation of the chemical compound is relevant.
Explanation: Chemical compounds, according to the law of constant proportions, are made of elements present in a fixed ratio by mass. The concentration of each element in a compound will always be the same by mass regardless of the source of the sample.
Nitrogen and oxygen atoms are always in a 1:2 ratio in the nitrogen dioxide molecule (NO2). Consequently, Nitrogen has the same structure as oxygen.
A Brief History of the Law of Definite Proportions
In the period between 1798-1804, French chemist Joseph Proust experimented with copper carbonate and water to develop a law of definite composition or proportions. Proust formulated his observations in what is now known as Proust's Law in 1806. As determined by mass, chemical compounds are composed of constant and defined ratios of elements. As an example, carbon dioxide consists of one carbon atom and two oxygen atoms. As a result, carbon dioxide can be described by the fixed ratio of 12 (mass of carbon):32 (mass of oxygen), or simplified as 3:8.
Disagreements with the Proust’s Law
Several chemists disagreed with Proust's theory at the time, particularly another French chemist, Claude Louis Berthollet. The French scientist believed that elements could mix in any ratio. A chemist called John Dalton proposed that chemical compounds were composed of atoms belonging to different elements. This idea was supported at an atomic level, however, as Dalton proposed that chemical compounds were composed of set formulas of atoms. In Dalton's law of multiple proportions, elements can combine to yield new combinations of elements in a compound. In such a scenario, the ratio of the elements within a compound can be expressed as a whole number, which is an extension of Dalton's law of definite composition.
The Law of definite proportions is not true universally, despite its considerable usefulness in modern chemistry. Different samples of a compound may have different elemental compositions due to non-stoichiometry. Compounds like these are subject to the Law of Multiple Proportions. As an example, the iron oxide wüstite, which may contain anywhere between 23 and 25 oxygen atoms by mass, holds 0.83 to 0.95 iron atoms for each oxygen atom. It is given as FeO, but the crystallographic vacancies result in FeO.95O being the ideal formula. The measurements of Proust were generally not accurate enough to detect these differences.
Furthermore, the composition of the element can differ depending on its source; therefore, the mass of the element can differ even within a pure, stoichiometric compound. Due to processing in the atmosphere, astronomy, crust, oceans, and deep Earth that tend to concentrate few environmental isotopes, one can use this variation in radiometric dating. Except for hydrogen and its isotopes, most of the time, the effect is small, but the instrumentation of today allows us to measure it.
Additionally, the composition of several natural polymers differs, even when they are considered "pure". As a rule, polymer molecules are not considered pure chemical compounds except when their molecular weights are uniform (which is mono distribution), and their stoichiometry is constant. They still might be in violation of the Law in these rare cases due to the isotopic variations.