the French chemist Henri Moissan
(1852–1907), who discovered it in
1893.
molal concentrationSee concen-
tration.
molalitySee concentration.
molar1.Denoting that an exten-
sive physical property is being ex-
pressed per *amount of substance,
usually per mole. For example, the
molar heat capacity of a compound is
the heat capacity of that compound
per unit amount of substance; in SI
units it would be expressed in J K–1
mol–1. 2.Having a concentration of
one mole per dm^3.
molar conductivity Symbol Λ.
The conductivity of that volume of
an electrolyte that contains one mole
of solution between electrodes
placed one metre apart.
molar heat capacity See heat ca-
pacity.
molaritySee concentration.
molar volume (molecular volume)
The volume occupied by a substance
per unit amount of substance.
mole Symbol mol. The SI unit of
amount of substance. It is equal to
the amount of substance that con-
tains as many elementary units as
there are atoms in 0.012 kg of car-
bon–12. The elementary units may
be atoms, molecules, ions, radicals,
electrons, etc., and must be speciÜed.
1 mole of a compound has a mass
equal to its relative molecular mass
expressed in grams.
molecular beamA beam of
atoms, ions, or molecules at low pres-
sure, in which all the particles are
travelling in the same direction and
there are few collisions between
them. They are formed by allowing a
gas or vapour to pass through an
aperture into an enclosure, which
acts as a collimator by containing
several additional apertures and vac-
uum pumps to remove any particles
that do not pass through the aper-
tures. Molecular beams are used in
studies of surfaces and chemical reac-
tions and in spectroscopy.molecular chaperone Any of a
group of proteins in living cells that
assist newly synthesized or dena-
tured proteins to fold into their func-
tional three-dimensional structures.
The chaperones bind to the protein
and prevent improper interactions
within the polypeptide chain, so that
it assumes the correct folded orienta-
tion. This process requires energy in
the form of ATP.
molecular distillationDistillation
in high vacuum (about 0.1 pascal)
with the condensing surface so close
to the surface of the evaporating liq-
uid that the molecules of the liquid
travel to the condensing surface
without collisions. This technique en-
ables very much lower temperatures
to be used than are used with distilla-
tion at atmospheric pressure and
therefore heat-sensitive substances
can be distilled. Oxidation of the dis-
tillate is also eliminated as there is
no oxygen present.
molecularÛow(KnudsenÛow)
TheÛow of a gas through a pipe in
which the mean free path of gas mol-
ecules is large compared to the di-
mensions of the pipe. This occurs at
low pressures; because most colli-
sions are with the walls of the pipe
rather than other gas molecules, the
Ûow characteristics depend on the
relative molecular mass of the gas
rather than its viscosity. The effect
was studied by M. H. C. Knudsen
(1871–1949).molecular formulaSee formula.
molecularityThe number of mol-
ecules involved in forming the acti-357 molecularity
m