The law of definite proportions means that when a particular compound undergoes a chemical reaction, the mass proportion of the substance involved in the reaction is always constant regardless of the origin of the substance.
This law is also known as Proust's law, law of constant proportion, law of multiple proportions, or the law of constant composition.
Furthermore, since elemental conversion does not occur in a chemical reaction, this also means that the various constituent elements have a fixed ratio by mass when they belong to a compound.
In more modern terms of the molecular formula, this law implies that fixed subscripts can permanently be assigned to each compound. Note that a class of compounds, called non-stoichiometric compounds (also called bertolides), do not follow this law. For these compounds, the ratio of the elements can vary continuously within certain limits.
Naturally, other materials such as alloys or colloids, which are not really compounds but mixtures, do not follow this law either.
It is called matter, everything that has mass and occupies a place in space. In most cases, matter can be perceived or measured by various analytical chemistry methods.
Who Proposed the Law of Definite Proportions?
Joseph Louis Proust published the law of definite proportions in 1797.
This law was implicitly recognized in quantitative analysis by other leading chemists in the eighteenth century and was made explicit and confirmed by the French chemist Joseph Louis Proust.
Joseph Proust conducted numerous investigations analyzing the composition of a wide variety of chemical compounds. With them, he discovered that the mass of each of the chemical elements maintained the same proportion of elements by mass before and after forming the compound.
Proust's law contradicted the conclusions of Claude Louis Berthollet (1748-1822). Berthollet believed that the elemental composition of the chemical elements in a compound depended on the formation conditions. This discrepancy generated an intense controversy at the beginning of the 19th century.
Proust supporters include Thomas Thomson, the English chemist John Dalton, who based Dalton's atomic theory on this law, and the Swedish chemist Jöns Jacob Berzelius.
In 1914, Russian chemist Nikolai Semenovich Kurnakov concluded that Proust's stoichiometric compounds should be considered particular types of phase classes, which he named daltonids after John Dalton.
However, intermetallic compounds and some metal oxides are known to change the ratio of component elements within a specific range and are called non-stoichiometric compounds or Beltré compounds.
An example is iron oxide, which can contain between 0.83 and 0.95 iron atoms for every oxygen atom.
Examples of the Law of Definite Proportions
For example, the mass ratio of hydrogen and oxygen that make up water is always 1: 8. Therefore, in the reaction 2 H 2 + O 2 → H 2 O. For each hydrogen atom, 8 grams of oxygen react.
Another example is that the numerical mass ratio of copper to oxygen that makes up copper(II) oxide is always 4:1.