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Density of Gases @ STP



Since gases are mostly empty space, the densities of gases are reported in g/L, not g/mL as found for solids and liquids. As you’re probably aware, density is equal to mass per unit of volume. To calculate the density of a gas at standard temperature and pressure, you take the molecular formula weight of the gas (grams per mole—from the periodic table) and divide that by the standard molar volume for a gas, which is 22.4 L per mole:

where the formula weight (FW) is in g/mol, and the standard molar volume is 22.4 L/mol. Now try using this in a problem.



What is the density of helium gas at STP?



If the density of the gas is equal to , then FW= 4.00 g/mol,  22.4 L/mol, so the density = 0.179 g/L.

Gas at STP

Density at STP (Highlight to reveal Answer) 
CO244g/22.4L = 1.96 g/L
H2S34g/22.4L = 1.51g/L
N228g/22.4L = 1.25 g/L

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