Definition and Related Concepts
As of 2011, the mole is defined by BIPM to be an amount of a substance that contains as many elementary entities (e.g. atoms, molecules, ions, electrons) as there are atoms in 12 grams of pure carbon-12 (12C), the isotope of carbon with atomic weight 12. Thus, by definition, one mole of pure 12C has a mass of exactly 12 g. It also follows from the definition that X moles of any substance will contain the same number of molecules as X moles of any other substance.
The mass per mole of a substance is called its molar mass. Since the standard unit for expressing the mass of molecules or atoms (the dalton or atomic mass unit) is defined as 1/12 of the mass of a 12C atom, it follows that the molar mass of a substance, measured in grams per mole, is exactly equal to its mean molecular or atomic mass, measured in daltons; which is to say, to the substance's mean molecular or atomic weight.
The number of elementary entities in a sample of a substance is technically called its (chemical) amount. Therefore, the mole is a convenient unit for that physical quantity. One can determine the chemical amount of a known substance, in moles, by dividing the sample's mass by the substance's molar mass. Other methods include the use of the molar volume or the measurement of electric charge.
It should be noted that the mass of one mole of a substance depends not only on its molecular formula, but also on the proportion of the isotopes of each element present in it. For example, one mole of calcium-40 is 39.96259098 ± 0.00000022 grams, whereas one mole of calcium-42 is 41.95861801 ± 0.00000027 grams, and one mole of calcium with the normal isotopic mix is 40.078 ± 0.004 grams.
Since the definition of the gram is not (as of 2011) mathematically tied to that of the dalton, the number NA of molecules in a mole (Avogadro's number) must be determined experimentally. The value adopted by CODATA in 2006 is NA = 6.02214179×1023 ± 0.00000030×1023. In 2011 the measurement was refined to 6.02214078×1023 ± 0.00000018×1023.
Read more about this topic: Mole (unit)
Famous quotes containing the words definition, related and/or concepts:
“The physicians say, they are not materialists; but they are:MSpirit is matter reduced to an extreme thinness: O so thin!—But the definition of spiritual should be, that which is its own evidence. What notions do they attach to love! what to religion! One would not willingly pronounce these words in their hearing, and give them the occasion to profane them.”
—Ralph Waldo Emerson (1803–1882)
“Becoming responsible adults is no longer a matter of whether children hang up their pajamas or put dirty towels in the hamper, but whether they care about themselves and others—and whether they see everyday chores as related to how we treat this planet.”
—Eda Le Shan (20th century)
“Science is a dynamic undertaking directed to lowering the degree of the empiricism involved in solving problems; or, if you prefer, science is a process of fabricating a web of interconnected concepts and conceptual schemes arising from experiments and observations and fruitful of further experiments and observations.”
—James Conant (1893–1978)