Amount of substance: symbol n, a quantity fundamental to
chemistry. Atoms and molecules are much too small or light to be counted or
weighed individually in the laboratory. The chemist therefore needs a unit to
specify the quantity amount of substance of an appropriate magnitude
(size) for laboratory or industrial scale work. The chosen unit is the mole.
Mole:
symbol mol, the unit of the quantity amount of substance. The mole is
defined as the amount of substance of a system which contains as many elementary
entities as there are atoms in 12 grams of carbon-12 (i.e. carbon consisting
only of the isotope 12C). 12 g is an easily measurable mass. When the mole
is used, the elementary entities must be specified and may be atoms, molecules,
ions, electrons, other particles or specified groups of such particles.
It follows from this definition of the mole that x
moles of dihydrogen (H2,) will contain exactly the same number of
dihydrogen molecules as there are dioxygen molecules (O2) in x moles
of dioxygen or water molecules (H2O) in x moles of water. Thus it follows from
the chemical equation
2H2 + O2 → 2H2O that 2 moles
of dihydrogen react with 1 mole of dioxygen to give 2 moles of water, i.e. the chemical
equation is also a simple way of expressing the reaction of measurable amounts
of substances as well as of individual molecules. Thus the chemical equation
C2H6 + 7/2O2 → 2CO2 + 3H2O
is quite satisfactory when the reaction coefficients
refer to amounts measured in moles.
Molar mass:
symbol M, the mass per mole of substance (the substance being defined by
its chemical formula). (Molar means per mole in this context.) The mass of any
substance is proportional to the amount of that substance and the
proportionality constant is its molar mass;
m =
Mn. [e.g. for water m = 18 g when n = 1 mol; 18 g = M(H2O) x 1 mol;
M(H2O