Molar Volume Calculator

22.414 L/mol at 0 °C and 1 atm, which is the older definition. Under the current IUPAC standard of 0 °C and 1 bar it is 22.711 L/mol — a 1.3% difference, so it matters which one a question means.

Because it belongs to the pre-1982 definition of STP that most textbooks were written against. IUPAC changed the standard pressure from 1 atm to 1 bar in 1982 and many teaching materials never followed.

Standard ambient temperature and pressure: 25 °C and 1 bar, where a mole occupies 24.790 L. It is closer to conditions in a real laboratory than 0 °C is.

Not for an ideal gas. Avogadro's law says equal volumes hold equal numbers of particles, so a mole of hydrogen and a mole of carbon dioxide occupy the same volume — even though one weighs twenty-two times the other.

At high pressure and low temperature, where molecular volume and intermolecular attraction stop being negligible. Near the boiling point, real molar volumes can be several percent off.

It converts moles of a gas straight to a volume without going through the ideal gas law. It is only a shortcut at the conditions it was quoted for, though — use PV = nRT for anything else.

Most school syllabuses still use 0 °C and 1 atm, giving 22.4 L/mol. IUPAC's current definition is 0 °C and 1 bar, giving 22.7. If a paper quotes 22.4 in its data booklet, it means the older definition.

Multiply by the pressure ratio. 22.414 × (101.325/100) = 22.711, which is the whole of the difference — the temperature is the same in both definitions and only the pressure changed.

Z = PV/nRT, which equals 1 for an ideal gas. Real gases deviate: Z below 1 means attraction dominates and the gas is more compressible than ideal; above 1 means molecular volume dominates. It is the standard way to quantify how far a gas departs from ideality.

It does not, strictly — molar volume is a shortcut for one set of conditions. Its value is as a sanity check: an answer implying 5 L/mol or 100 L/mol at ordinary conditions has an error in it somewhere.

The term does, as molar mass divided by density, but the number is completely different. Liquid water is about 18 mL/mol against 22,400 mL/mol for steam at STP — a factor of over a thousand, which is why boiling is so dramatic a volume change.

Those with strong intermolecular forces or large molecules. Ammonia and water vapour hydrogen-bond and deviate noticeably; carbon dioxide is intermediate; helium and hydrogen are nearly ideal at ordinary conditions because their attractions are so weak.