Mole
Mole is a unit of measurement used in chemistry to
express amounts of a chemical substance, defined as the amount of any
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 relative atomic mass 12. This corresponds to
the Avogadro constant, which has a value of
6.02214179(30)×1023 elementary entities of the substance. It is one
of the base
units in the International System of Units, and
has the unit symbol mol and corresponds with
the dimension symbol N. In honor of the unit, chemists often celebrate October 23 (a reference to the
1023 part of Avogadro's number) as "Mole Day".
The mole is widely used in chemistry instead of units of
mass or volume as a convenient way to express amounts of reactants or of
products of chemical reactions. For example, the chemical equation 2 H2
+ O2 → 2 H2O implies that 2 mol of dihydrogen
(H2) and 1 mol of dioxygen (O2)
react to form 2 mol of water (H2O). The mole may also be used
to express the number of atoms, ions, or other elementary entities in a given
sample of any substance. The concentration
of a solution is commonly expressed by its molarity,
defined as the number of moles of the dissolved substance per litre of solution.
The number of molecules in a mole (known as Avogadro's
number) is defined such that the mass of one mole of a substance, expressed in
grams, is exactly equal to the substance's mean molecular
mass. For example, the mean molecular mass of natural water is about 18.015, so one mole of water is
about 18.015 grams. Making use of this equation considerably simplifies
many chemical and physical computations.
The term gram-molecule was formerly used for
essentially the same concept. The term gram-atom (abbreviated gat.)
has been used for a related but distinct concept, namely a quantity of a
substance that contains Avogadro's number of atoms, whether isolated or
combined in molecules. Thus, for example, 1 mole of MgB2 is
1 gram-molecule of MgB2 but 3 gram-atoms of MgB2.
http://xenontics.blogspot.com/