Molar volume
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), although it is more practical to use the units cubic decimetres per mole (dm3/mol) for gases and cubic centimetres per mole (cm3/mol) for liquids and solids.
The molar volume of a substance can be found by measuring its molar mass and density then applying the relation
If the sample is a mixture containing N components, the molar volume is complex. It can be simply the sum of the individual components, and calculated using:
But this is violated by many liquid-liquid mixtures. For instance mixing pure ethanol into pure water causes a decrease in the volume calculated by this formula. This effect is called "excess volume".
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:
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 = 0.08206 L atm K−1 mol−1, that is a relative standard uncertainty of 9.1×10−7, according to the 2010 CODATA recommended value. The molar volume of an ideal gas at 100 kPa (1 bar) is
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