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# How do I estimate gas densities at STP?

How do I find the densities of oxygen , methane and carbon dioxide gases at STP?
Najam Mirza

Density is mass over volume. To solve the problem, you have to find the mass of a sample of unit volume. Here is a general strategy for getting a quick and dirty estimate of gas densities:

1. Find the molar volume. At STP, one mole of an ideal gas will occupy 22.414 L. At other temperatures and pressures, the molar volume of an ideal gas is V/n = RT/P. If you can't assume the gas is ideal, you'll need to either look up the molar volume in a handbook or use a real-gas equation of state to get it.
2. Find the molar mass. You know the mass of one mole of each gas from its molecular weight.
3. Density is mass over volume, so dividing the molar mass by the molar volume gives you the density. Gas densities are somewhere around 1 g/L at room temperature and pressure- about 1/1000th the density of liquids.

Note that you can very quickly decide which gas is going to be densest. If they behave ideally, all have the same molar volume at STP and the densest gas will be the one with the highest molar mass.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! How do I estimate gas densities at STP?