Molar volume
Molar volume | |
---|---|
Common symbols | Vm, |
SI unit | m3/mol |
Other units | dm3/mol, cm3/mol |
Dimension | L3 N−1 |
In chemistry and related fields, the molar volume, symbol Vm,[1] or of a substance is the ratio of the volume occupied by a substance to the amount of substance, usually at a given temperature and pressure. It is equal to the molar mass (M) divided by the mass density (ρ):
The molar volume has the SI unit of cubic metres per mole (m3/mol),[1] although it is more typical to use the units cubic decimetres per mole (dm3/mol) for gases, and cubic centimetres per mole (cm3/mol) for liquids and solids.
Definition[edit]
The molar volume of a substance i is defined as its molar mass divided by its density ρi0:
Relation to specific volume[edit]
Molar volume is related to specific volume by the product with molar mass. This follows from above where the specific volume is the reciprocal of the density of a substance:
Ideal gases[edit]
For ideal gases, the molar volume is given by the ideal gas equation; this is a good approximation for many common gases at standard temperature and pressure. The ideal gas equation can be rearranged to give an expression for the molar volume of an ideal gas:
The molar volume of an ideal gas at 100 kPa (1 bar) is
- 0.022710954641485... m3/mol at 0 °C,
- 0.024789570296023... m3/mol at 25 °C.
The molar volume of an ideal gas at 1 atmosphere of pressure is
- 0.022413969545014... m3/mol at 0 °C,
- 0.024465403697038... m3/mol at 25 °C.
Crystalline solids[edit]
For crystalline solids, the molar volume can be measured by X-ray crystallography. 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
Molar volume of silicon[edit]
Ultra-pure silicon is 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 in 1974.[2] The interest stems from 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.[3]
The CODATA recommended value for the molar volume of silicon is 1.205883199(60)×10−5 m3⋅mol−1, with a relative standard uncertainty of 4.9×10−8.[4]
See also[edit]
References[edit]
- ^ a b International Union of Pure and Applied Chemistry (1993). Quantities, Units and Symbols in Physical Chemistry, 2nd edition, Oxford: Blackwell Science. ISBN 0-632-03583-8. p. 41. Electronic version.
- ^ Deslattes, R. D.; Henins, A.; Bowman, H. A.; Schoonover, R. M.; Carroll, C. L.; Barnes, I. L.; Machlan, L. A.; Moore, L. J.; Shields, W. R. (1974). "Determination of the Avogadro Constant". Phys. Rev. Lett. 33 (8): 463–66. Bibcode:1974PhRvL..33..463D. doi:10.1103/PhysRevLett.33.463.
- ^ Mohr, Peter J.; Taylor, Barry N. (1999). "CODATA recommended values of the fundamental physical constants: 1998" (PDF). Journal of Physical and Chemical Reference Data. 28 (6): 1713–1852. Bibcode:1999JPCRD..28.1713M. doi:10.1063/1.556049. Archived from the original (PDF) on 2017-10-01.
- ^ "2022 CODATA Value: molar volume of silicon". The NIST Reference on Constants, Units, and Uncertainty. NIST. May 2024. Retrieved 2024-05-18.