Molar volume

In chemistry and related fields, the molar volume, symbol Vm, or 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): The molar volume has the SI unit of cubic metres per mole (m3/mol), 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. The molar volume of a substance i is defined as its molar mass divided by its density ρi0: 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: 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. 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: 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 based on the gas constant: R = 8.31446261815324m3⋅Pa⋅K−1⋅mol−1, or about 8.20573660809596e-5m3⋅atm⋅K−1⋅mol−1. The molar volume of an ideal gas at 100 kPa (1 bar) is 0.022710954641485m3/mol at 0 °C, 0.024789570296023m3/mol at 25 °C. The molar volume of an ideal gas at 1 atmosphere of pressure is 0.022413969545014m3/mol at 0 °C, 0.024465403697038m3/mol at 25 °C. For crystalline solids, the molar volume can be measured by X-ray crystallography.