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Avogadro, (Lorenzo Romano) Amedio (Carlo)

law mole gases atoms

[avoh ga droh] (1776–1856) Italian physicist: proposed a method for finding molecular formulae of gases.

Trained in law like his forefathers and working as a lawyer for some time, after 1800 he turned to science and held professorships in physics for much of his life. His fame now rests on one brilliant and important idea. He considered Law of combining volumes and with little evidence offered a daring explanation for it in 1811. His idea, Avogadro’s Law, was that ‘equal volumes of all gases, under the same conditions of temperature and pressure, contain the same number of smallest particles’. There is now ample evidence that he was right; in some cases (eg the noble gases) the smallest particles are atoms; for most other gases, they are combinations of atoms (molecules). The law gives a direct method of finding the molecular formula of a gas, and such a formula in turn gives the relative atomic masses of the elements present in it. Avogadro’s Law shows that the simple gases hydrogen and oxygen are diatomic (H2 and O2 ) and that water is H2O (and not HO as believed). However, the law was largely rejected or ignored for 50 years (although accepted it) until in 1860 convinced a Chemical Congress at Karlsruhe of its value.

The SI base unit of amount of substance is the mole (which is related to Avogadro’s Law). The mole is defined as containing as many elementary entities (usually atoms or molecules, and specified for each case) as there are atoms in 0.012 kg of carbon-12. Thus for a compound, 1 mole has a mass equal to its relative molecular mass in grams. The number of entities in a mole, the Avogadro constant, N A , is 6.022× 10 23 mol –1 ; and 1 mole of any ideal gas, at STP (standard temperature and pressure), has a molar volume of 22.415 dm 3 .

For example: since the relative atomic masses (‘atomic weights’) of carbon and oxygen are 12 and 16 respectively, a mole of carbon dioxide (CO 2 ) will weigh 12+(2×16)=44g, and will have a volume at STP close to 22.4 dm 3 .

Avshalomov, Jacob (David) [next] [back] Avni, Tzvi (Jacob)

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