Molar volume
Common symbols	Vm, ${\displaystyle {\tilde {V}}}$
SI unit	m3/mol
Other units	dm3/mol, cm3/mol
Dimension	L3 N−1

In chemistry and related fields, the molar volume, symbol V_m,^[1] or ${\displaystyle {\tilde {V}}}$ of a substance is the ratio of the volume occupied by a substance to the amount of substance, usually given at a given temperature and pressure. It is equal to the molar mass (M) divided by the mass density (ρ):

$${\displaystyle V_{\text{m}}={\frac {M}{\rho }}}$$

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

Change in volume with increasing ethanol fraction.

The molar volume of a substance i is defined as its molar mass divided by its density ρ_i⁰:

$${\displaystyle V_{\rm {m,i}}={M_{i} \over \rho _{i}^{0}}}$$

For an ideal mixture containing N components, the molar volume of the mixture is the weighted sum of the molar volumes of its individual components. For a real mixture the molar volume cannot be calculated without knowing the density:

$${\displaystyle V_{\rm {m}}={\frac {\displaystyle \sum _{i=1}^{N}x_{i}M_{i}}{\rho _{\mathrm {mixture} }}}}$$

There are many liquid–liquid mixtures, for instance mixing pure ethanol and pure water, which may experience contraction or expansion upon mixing. This effect is represented by the quantity excess volume of the mixture, an example of excess property.

Relation to specific volume

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:

$${\displaystyle V_{\rm {m,i}}={M_{i} \over \rho _{i}^{0}}=M_{i}v_{i}}$$

Ideal gases

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:

$${\displaystyle V_{\rm {m}}={\frac {V}{n}}={\frac {RT}{P}}}$$

Hence, for a given temperature and pressure, the molar volume is the same for all ideal gases and is based on the gas constant: R = 8.31446261815324 m³⋅Pa⋅K⁻¹⋅mol⁻¹, or about 8.20573660809596×10⁻⁵ m³⋅atm⋅K⁻¹⋅mol⁻¹.

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

For crystalline solids, the molar volume can be measured by X-ray crystallography. The unit cell volume (V_cell) 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

$${\displaystyle V_{\rm {m}}={{N_{\rm {A}}V_{\rm {cell}}} \over {Z}}}$$

where N_A 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".

See also

• Specific volume
• Standard temperature and pressure#Molar volume of a gas

References

External links

• Interactive table of molar volumes