Molar volume

In chemistry, the molar volume V_m of a substance is the ratio of the volume V of a sample of that substance to the number of moles n in the sample: V_m = V/n. Apart from computing molar volume by dividing the volume of the substance by the number of moles of substance, it can also be computed as the substance's molar mass divided by its density. Indeed, if the molar mass of substance X is M(X) [kg/mol] and the density of a sample of X is ρ [kg/m³], then the molar volume of X is given by V_m = M(X)/ρ [m³/mol].

The SI unit of molar volume is cubic metres per mole (m³/mol). It is the reciprocal of amount of substance concentration (moles per volume).

For an ideal gas, the standard molar volume is the volume that is occupied by one mole of substance (in gaseous form) at standard temperature and pressure (STP) of 273.15 K (0C) and 101325 Pa (1 atmosphere or 760 torr). It is 0.022413 m³mol^-1 or 22.413 L/mol and is directly related to the universal gas constant R in the ideal gas law.

The molar volume for an ideal gas at 298.15 K and 1 bar is 0.024788 m³mol^-1 or 24.788 L/mol.

Cubic centimetres (cm³), a measure of volume one million times smaller than a cubic metre, are sometimes also used, to give units of cm³mol^-1.

The molar volume is usually given for a solid substance at 298.15 K (temperature of standard state). Apart from temperature and density, it depends on phase and allotrope of the substance.

See also

• Concentration