In chemistry, the law of definite proportions, sometimes called Proust`s law or law of constant composition, states that a given chemical compound always contains its constituents in a fixed ratio (by mass) and does not depend on its source and method of production. For example, oxygen makes up about 8/9 of the mass of a pure water sample, while hydrogen makes up the remaining 1/9 of the mass: the mass of two elements in a compound is always in the same proportion. Together with the law of multiple proportions, the law of defined proportions forms the basis of stoichiometry. [1] Many natural polymers differ in composition (e.g., DNA, proteins, carbohydrates), even if they are “pure.” Polymers are generally not considered “pure chemical compounds” unless their molecular weight is uniform (monodisperse) and their stoichiometry is constant. In this unusual case, they may still break the law due to isotopic variations. The law of defined proportions states that all samples of a single compound contain the same mass fraction of elements. Other names for the law are Proust`s law or the law of constant composition. Whatever you call it, this law, together with the law of multiple proportions, is the basis of stoichiometry in chemistry. The law of constant composition states that in a given chemical compound, all samples of that compound consist of the same elements in the same ratio or ratio. For example, each water molecule always consists of two hydrogen atoms and one oxygen atom in a ratio (2:1).

If we look at the relative masses of oxygen and hydrogen in a water molecule, we see that (text{94}%) of the mass of a water molecule is oxygen, and the rest (text{6}%) is the mass of hydrogen. This mass fraction is the same for each water molecule. Although very useful in the basis of modern chemistry, the law of certain proportions is not universally true. There are non-stoichiometric compounds whose elemental composition may vary from one sample to another. Such connections follow the law of multiple actions. An example is iron oxide wüstite, which can contain between 0.83 and 0.95 iron atoms for each oxygen atom and therefore contains between 23% and 25% oxygen by mass. The ideal formula is FeO, but due to crystallographic voids, it is about Fe0.95O. In general, Proust`s measurements were not accurate enough to detect such variations.

The law of constant compositions and the law of multiple proportions are theories in physical chemistry used to explain stoichiometry in chemistry. Stoichiometry is the measurement of the relative amounts of reactants and products in a chemical reaction. In addition, the isotopic composition of an element can vary depending on the source, so its contribution to the mass of a pure stoichiometric compound can vary. This variation is used in radiometric dating because astronomical, atmospheric, oceanic, crustal and deep processes can preferentially concentrate certain environmental isotopes. With the exception of hydrogen and its isotopes, the effect is generally small, but measurable with modern instruments. The law of constant composition states that when elements are chemically combined, they are always present in certain proportions of weight. This is called the law of constant composition. In any chemical compound, the elements always combine in the same proportion to each other. This is the law of constant composition. There is no clear formula for the composition of a mixture. If you want to mix a spoonful of sugar with your tea, you can; If you want to mix two, you. A pinch of sulfur can be mixed, a pound of iron filings or a pound of sulfur can be mixed with a pinch of iron filings.

It is possible to prepare mixtures of complete compositions. But that certainly doesn`t apply to connections. For example, iron sulfide, the compound of iron and sulfur, always contains 56 grams of iron to 32 grams of sulfur. By weight, iron sulfide contains 36.4% sulfur and 63.6% iron. Provided that the composition is pure, its composition will always be the same. The credit for the discovery of the law goes to the French chemist Joseph Proust, who described it in 1797. Joseph Priestley and Antoine Lavoisier also observed how the elements combine in constant proportions. The law can be revised by producing two samples of copper oxide, each prepared by a different method, and then calculating the composition of each sample (see figure). In both cases, metallic copper is the starting point.

It can be converted directly to oxide by intense heating in air or dissolved in concentrated nitric acid to convert it to nitrate. Strong heating breaks down nitrate and expels oxygen and nitrogen oxides. The copper oxide remains. The composition of the two copper oxide samples can be calculated and detected as equal. Its composition is constant and does not depend on the method of production. There are exceptions to the law of certain proportions. The mass composition of non-stoichiometric compounds varies from sample to sample. For example, the formula for the iron oxide mineral wustite is Fe0.95O, not FeO, because the crystal structure allows for slight deviations from the stoichiometric ratio. The discovery of this law is attributed to the French chemist Joseph Proust, who, through a series of experiments conducted from 1798 to 1804, concluded that chemical compounds consist of a certain composition. Considering that John Dalton`s atomic theory was just beginning to explain that each element consisted of some kind of atom, and that at the time most scientists still believed that elements could combine in any report, Proust`s conclusions were extraordinary. In 1794, the French chemist Joseph Proust formulated the law of constant proportions from his work on sulphides, metal oxides and sulphates. This law met with much resistance in the scientific community in the 18th century.

The introduction of Dalton`s atomic theory favored this law and a relationship between these two concepts was established by the Swedish chemist Jacob Berzelius in 1811. The law of constant proportions states that chemical compounds consist of elements present in a fixed mass ratio. This means that any pure sample of a compound, regardless of the source, always consists of the same elements present in the same mass ratio. For example, pure water always contains hydrogen and oxygen in a fixed mass ratio (one gram of water consists of about 0.11 grams of hydrogen and 0.88 grams of oxygen, the ratio is 1: 8). The main difference between the law of constant composition and the law of multiple fractions is that, according to the law of constant composition, the same proportion of samples always consists of the same mass fraction, whereas according to the law of multiple proportions, if two elements are connected to each other and form more than one chemical compound, then the ratio between the masses of the second element, which combines with a fixed mass of the first element, ratios of small integers. It turns out that there are exceptions to this rule. Some non-stoichiometric compounds have a variable composition from one sample to another. An example is wusite, a type of iron oxide that can contain 0.83 to 0.95 iron per oxygen.