Molar MassMolar mass of gases: volume and density
For a gas, molar mass tells you more than how much to weigh. It tells you how dense the gas is, whether it rises or sinks in air, and how fast it spreads.
One mole of any gas takes up the same space
Avogadro's law says that equal volumes of gases at the same temperature and pressure hold equal numbers of molecules. So one mole of any gas that behaves ideally occupies the same volume, called the molar volume.
- At 0 °C and 1 atmosphere pressure the molar volume is 22.4 litres. This is the value used in most school books in India.
- At 0 °C and 1 bar, the standard used by IUPAC, it is 22.7 litres.
- At room temperature, about 25 °C, and 1 atmosphere, it is roughly 24.5 litres.
Use the value your syllabus or question states.
Density of a gas
density at 0 °C and 1 atm = molar mass ÷ 22.4 (in grams per litre)
| Gas | Molar mass, g/mol | Density, g/L |
|---|---|---|
| Hydrogen H2 | 2.02 | 0.090 |
| Helium He | 4.00 | 0.179 |
| Methane CH4 | 16.04 | 0.716 |
| Ammonia NH3 | 17.03 | 0.760 |
| Nitrogen N2 | 28.01 | 1.250 |
| Oxygen O2 | 32.00 | 1.428 |
| Carbon dioxide CO2 | 44.01 | 1.963 |
| Chlorine Cl2 | 70.90 | 3.163 |
Lighter or heavier than air?
Air is a mixture, mostly nitrogen and oxygen, with an average molar mass of about 29 g/mol. A gas with a molar mass below 29 is lighter than air and rises; one above 29 is heavier and sinks.
- Hydrogen, helium, methane and ammonia are lighter. Ammonia is collected in an upturned jar.
- Carbon dioxide, chlorine, sulphur dioxide and hydrogen chloride are heavier. They are collected in an upright jar, and carbon dioxide can be poured over a candle to put it out.
- LPG, mainly propane (44) and butane (58), is heavier than air. A leak collects near the floor, which is why the advice is to open doors and windows and not to switch anything on.
Vapour density
Vapour density compares a gas with hydrogen: it is the mass of a volume of the gas divided by the mass of the same volume of hydrogen under the same conditions. Because hydrogen has a molar mass of about 2,
molar mass = 2 × vapour density
A gas with a vapour density of 22 has a molar mass of 44; it could be carbon dioxide. A vapour density of 8 gives 16: methane.
Volume, moles and mass
Example. What is the mass of 5.6 litres of oxygen at 0 °C and 1 atm? Moles = 5.6 ÷ 22.4 = 0.25. Mass = 0.25 × 32.00 = 8.00 g.
Example. What volume does 11 g of carbon dioxide occupy at 0 °C and 1 atm? Moles = 11 ÷ 44.01 = 0.250. Volume = 0.250 × 22.4 = 5.6 litres.
How fast gases spread
Graham's law says that lighter gases diffuse faster: the rate is inversely proportional to the square root of the molar mass. Hydrogen, with a molar mass of 2, diffuses four times as fast as oxygen, with 32, because the square root of 16 is 4. In the classroom demonstration with ammonia and hydrogen chloride at opposite ends of a tube, the white ring forms nearer the hydrogen chloride end, because the lighter ammonia travels further.
The gases on this site are gathered on the elements and common gases page.
Which values? This site uses the abridged standard atomic weights published by IUPAC (CIAAW), read on 12 November 2026. Your textbook or question paper may round them differently, for example H = 1 and O = 16, so its answer can differ slightly. In an exam, use the values the paper gives.