The molar volume, symbol V_{m},^{[1]} 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 (m^{3}/mol),^{[1]} although it is more practical to use the units cubic decimetres per mole (dm^{3}/mol) for gases and cubic centimetres per mole (cm^{3}/mol) for liquids and solids.
The molar volume of a substance can be found by measuring its mass and density then applying the relation
If the sample is a mixture containing N components, the molar volume is calculated using:
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. For crystalline solids, the molar volume can be measured by Xray crystallography.
Contents
Ideal gases
The ideal gas equation can be rearranged to give an expression for the molar volume of an ideal gas:
Hence, for a given temperature and pressure, the molar volume is the same for all ideal gases and is known to the same precision as the gas constant: R = 8.314 472(15) J mol^{–1} K^{–1}, that is a relative standard uncertainty of 1.7×10^{–6}, according to the 2006 CODATA recommended value.^{[2]} The molar volume of an ideal gas at 100 kPa (1 bar) is
The molar volume of an ideal gas at 1 atmosphere of pressure is
mols are measured n=m/M n=mols m=mass M=molar mass
Crystalline solids
The unit cell volume (V_{cell}) may be calculated from the unit cell parameters, whose determination is the first step in an Xray crystallography experiment (the calculation is performed automatically by the structure determination software). This is related to the molar volume by
Full article ▸
