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In chemistry, the law of definite proportions, sometimes called Proust's law or the law of definite composition, states that a chemical compound always contains exactly the same proportion of elements by mass. An equivalent statement is the law of constant composition, which 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. Along with the law of multiple proportions, the law of definite proportions forms the basis of stoichiometry.〔Zumdahl, S. S. “Chemistry” Heath, 1986: Lexington, MA. ISBN 0-669--04529-2.〕 ==History== This observation was first made by the French chemist Joseph Proust, based on several experiments conducted between 1798 and 1804.〔For example: Proust, J.-L. (1799). Researches on copper, ''Ann. chim.'', 32:26-54. (Excerpt ), in Henry M. Leicester and Herbert S. Klickstein, ''A Source Book in Chemistry, 1400-1900'', Cambridge, MA: Harvard, 1952. Accessed 2008-05-08.〕 Based on such observations, Proust made statements like this one, in 1806: The law of definite proportions might seem obvious to the modern chemist, inherent in the very definition of a chemical compound. At the end of the 18th century, however, when the concept of a chemical compound had not yet been fully developed, the law was novel. In fact, when first proposed, it was a controversial statement and was opposed by other chemists, most notably Proust's fellow Frenchman Claude Louis Berthollet, who argued that the elements could combine in any proportion.〔See Dalton, J. (1808). ''op. cit.'', ch. II, that Berthollet held the opinion that in all chemical unions, there exist insensible gradations in the proportions of the constituent principles.〕 The existence of this debate demonstrates that, at the time, the distinction between pure chemical compounds and mixtures had not yet been fully developed.〔Proust argued that ''compound'' applies only to materials with fixed proportions: Proust, J.-L. (1806). Sur les mines de cobalt, nickel et autres, ''Journal de Physique'', 63:566-8. (Excerpt ), from Maurice Crosland, ed., ''The Science of Matter: a Historical Survey'', Harmondsworth, UK: Penguin, 1971. Accessed 2008-05-08.〕 The law of definite proportions contributed to, and was placed on a firm theoretical basis by, the atomic theory that John Dalton promoted beginning in 1803, which explained matter as consisting of discrete atoms, that there was one type of atom for each element, and that the compounds were made of combinations of different types of atoms in fixed proportions.〔Dalton, J. (1808). ''A New System of Chemical Philosophy, volume 1'', Manchester. (Excerpt ). Accessed 2008-05-08.〕 A related early idea was Prout's hypothesis, formulated by English chemist William Prout, who proposed that the hydrogen atom was the fundamental atomic unit. From this hypothesis was derived the whole number rule, which was the rule of thumb that atomic masses were whole number multiples of the mass of hydrogen. This was later rejected in the 1820s and 30s following more refined measurements of atomic mass, notably by Jöns Jacob Berzelius, which revealed in particular 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 any isotope is very close to satisfying the whole number rule, with the mass defect caused by differing binding energies being significantly smaller. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Law of definite proportions」の詳細全文を読む スポンサード リンク
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