Everything about The Molar Volume totally explained
The
molar volume, symbol
Vm, is the
volume occupied by one
mole of a substance (
chemical element or
chemical compound) at a given
temperature and
pressure. It is equal to the
molar mass (
M) divided by the
mass density (ρ). It has the
SI unit cubic metres per mole (m³/mol), The molar volume of an ideal gas at 100
kPa (1
bar) is
» 22.710 980(38) dm³/mol at 0 °C
24.789 598(42) dm³/mol at 25 °C
Crystalline solids
The
unit cell volume (
Vcell) may be calculated from the
unit cell parameters, whose determination is the first step in an
X-ray crystallography experiment (the calculation is performed automatically by the structure determination software). This is related to the molar volume by
» :
where
NA is the
Avogadro constant and
Z is the number of formula units in the unit cell. The result is normally reported as the "crystallographic density".
Molar volume of silicon
High quality single crystals of ultrapure
silicon are routinely made for the electronics industry, and the measurement of the molar volume of silicon, both by
X-ray crystallography and by the ratio of
molar mass to
mass density, has attracted much attention since the pioneering work at
NIST by Deslattes
et al. (1974). The interest stems from the fact that accurate measurements of the unit cell volume,
atomic weight and mass density of a pure crystalline solid provide a direct determination of the
Avogadro constant. At present (2006
CODATA recommended value), the precision of the value of the Avogadro constant is limited by the
uncertainty in the value of the
Planck constant (relative standard uncertainty of 5×10
–8).
The 2006 CODATA recommended value for the molar volume of silicon is 12.058 8349(11)×10
–6 m³/mol, with a relative standard uncertainty of 9.1×10
–8.
Further Information
Get more info on 'Molar Volume'.
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