The molar volume is the volume occupied by one mole of any gas under stated conditions; it is 24 dm3 mol−1 at room conditions and 22.4 dm3 mol−1 at STP.
The molar volume is the volume occupied by one mole of any gas under stated conditions. At room conditions it is 24 dm3 mol−1, and at standard temperature and pressure it is 22.4 dm3 mol−1. Malay: isi padu molar · Chinese: 摩尔体积.
The striking idea here is that one mole of any gas takes up the same volume under the same conditions, whatever the gas is, because the size of gas particles is negligible compared with the space between them. This follows directly from Avogadro’s law, and it lets you convert between the amount of a gas in moles and its volume without knowing its molar mass.
Example. At room conditions, two moles of carbon dioxide occupy 2 × 24 dm3 mol−1 = 48 dm3, while half a mole of hydrogen occupies 12 dm3. Working the other way, if a gas fills 6 dm3 at room conditions, then it must be a quarter of a mole. To find the number of moles of a gas, you divide its volume by the molar volume for the stated conditions, and to find a volume you multiply the moles by the molar volume instead.
Confusion to avoid. Molar volume applies only to gases, not to solids or liquids, whose one-mole volumes vary from substance to substance. Be careful to use the value that matches the conditions given in the question: 24 dm3 mol−1 at room conditions but 22.4 dm3 mol−1 at STP. Do not confuse molar volume with molar mass, which is about grams rather than volume.
In this chapter the molar volume is the gas counterpart of molar mass: it is the third route into the mole, alongside mass and number of particles. Because so many reactions produce or use up gases, it turns up in a great many calculations across SPM Chemistry, so it repays learning both values and the conditions each one belongs to.
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