A related first idea was the Prout hypothesis, formulated by the English chemist William Prout, who proposed that the hydrogen atom was the basic atomic unit. From this assumption, the rule of integers was derived, which was the rule of thumb that atomic masses are integer multiples of the mass of hydrogen. This was later rejected in the 1820s and 30s after refined measurements of atomic mass, in particular by Jöns Jacob Berzelius, who notably revealed that the atomic mass of chlorine was 35.45, which was incompatible with the hypothesis. Since the 1920s, this discrepancy has been explained by the presence of isotopes; The atomic mass of an isotope is very close to satisfying the rule of integers,[5] the mass defect caused by different binding energies being significantly smaller. 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. The samples follow the law of constant composition, which allows significant numbers and experimental errors. Regardless of the amount of reagents added, the same products with the same compositions are formed (i.e. the precipitate observed in the reactions). However, if the reagents are not added in the correct proportions, there are still reagents that have not reacted in the final solution with the products formed.
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] On September 26, 1754, the French chemist Joseph Louis Proust was born. He is best known for his discovery of the law of constant composition in 1799, which states that matter is neither created nor destroyed in chemical reactions. 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. Although Proust was right in his observations, the reason why reactants behave as he described them only became clear when the English chemist John Dalton formulated his atomic theory in 1803. According to Dalton, a fixed number of atoms of one substance always combines with a fixed number of atoms of another substance to form a compound. Dalton realized that substances must combine in the same proportions by weight as the weight proportions of their atoms.
Other chemists had already observed that pure substances combine in fixed proportions. They called finding the law of certain proportions (or constants). Dalton`s theory explained the law. 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. In addition, the isotopic composition of an element can vary depending on the source, so its weight can vary in a pure stoichiometric compound. This fact is used in geochemical dating because the astronomical, atmospheric, oceanic, crustal and deep processes of the Earth can preferentially concentrate lighter or heavier isotopes.
With the exception of hydrogen and its isotopes, the effect is generally small, but measurable with modern instruments. An additional note: Many natural polymers vary in their composition (e.g., RNA, 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.