Molar Mass of a Gas Calculator
Weigh it, measure its volume, and the ideal gas law identifies it — how molar masses were found before mass spectrometry.
The formula
M = m R T / (P V)
Weighing a gas to identify it
Rearranging the ideal gas law puts the molar mass in terms of four things you can measure with a balance, a barometer, a thermometer and a flask. This is how molar masses were determined before mass spectrometry, and it is still the cleanest demonstration that the mole is a countable quantity.
The molar volume is not 22.4 litres
It is 22.414 at 0 °C and one atmosphere, the old definition of STP. IUPAC changed the reference pressure to 100 kPa in 1982, which makes the molar volume 22.711 L — and at room temperature it is nearer 24.5. The number worth remembering is the equation, not any particular value of it.
Where the ideal law fails
Real gases depart from it at high pressure and near their condensation point, because molecules occupy volume and attract one another. For gases well above their boiling points at ordinary pressures the error is well under a per cent, which is why the approximation survives so much use.
Empirical is not molecular
Every method on these pages returns a ratio, because that is all a composition can determine. Glucose, formaldehyde and acetic acid all reduce to CH₂O, and no amount of elemental analysis will separate them. A molar mass from mass spectrometry or a freezing-point depression is what turns the ratio into a formula, and it has to come from a second, independent measurement.