Molar volume of a gas
The volume of a gas is sensitive to both pressure and temperature. Because a gas is easily compressible, by adjusting the pressure and temperature of a gas sample, the volume that the gas sample occupies can be altered. Therefore, if the volumes of any two gas samples are to be compared, they must be under the same conditions (ie - the two gas samples MUST be at the same temperature and pressure).
A common set of conditions that is often used by scientists is called Standard Temperature and Pressure or STP. When the STP condition is implied, scientists know that the gas must be at a specific temperature and a specific pressure. The conditions used for reference are:
- standard temperature is 273 K (or 0oC).
- standard pressure is 1 atm (760 torr)
Gas densities at STP
For any gas at STP, when the molar mass of the gas is divided by the density of the gas, the value "22.4" is always obtained. Take a look at the "special" number, "22.4".
Or,
density of a gas = molar mass / molar volume
| Gas | Molar Mass (g/mole) |
Density at STP (g/L) |
Molar Mass/Density (L/mole) |
| N2, nitrogen gas | 28.014 | 1.25 | 22.4 |
| O2, oxygen gas | 31.998 | 1.43 | 22.4 |
| CH4, methane gas | 16.043 | 0.714 | 22.4 |
| CO2, carbon dioxide | 44.009 | 1.96 | 22.4 |
| H2, hydrogen gas | 2.015 | 0.090 | 22.4 |
| Ar, argon gas | 39.948 | 1.78 | 22.4 |
The molar volume of any gas at STP is 22.4 L.
The unit of 22.4 is L/mole.
Molar volume calculations
Using conversion factors, we can solve questions involving conversion:
from moles of gas to volume occupied at STP 
|
× 22.4 L/mole |
| from volume occupied at STP to moles of gas
|
÷ 22.4 L/mole |
External link
How to calculate gas density from the molar volume under STP conditions.
Content suitability
BCIT courses: CHEM 0011
