# What is the molar volume of gases at STP?

The molar volume of an ideal gas at STP is 22.71 L.

NOTE: Most text books and websites list the molar volume of a gas as 22.4L. That value is based on the STP values which were used before 1982 (273.15K and 101.3 kPa). The correct value of 22.71L is based on the IUPAC definition of STP being 273.15K and 100 kPa which has been used since the change in 1982. The web sites and books that list 22.4 are just not posting the most current (and correct) information. Try substituting 101.3 kPa into Ernest’s work and you’ll see that this is true.

To determine the volume of an ideal gas, we can use the .

##PV = nRT##

##V = (nRT)/P##

##n = “1 mol”##
##R = “8.314 kPa·L·K”^(-1)”mol”^(-1)##
##”STP” = “100 kPa”## and ##”273.15 K”##

##V = (nRT)/P = (1 cancel(“mol”) × 8.314 cancel(“kPa”)·”L”·cancel(“K⁻¹mol⁻¹”) × 273.15 cancel(“K”))/(100 cancel(“kPa”)) = “22.71 L”##

The molar volume of an ideal gas at STP is 22.71 L.

In practice, no gas is ideal.

Every real gas has molecules with a definite size and with attractive forces between them.

Even helium, which you would expect to be the most ideal, has a molar volume of 22.73 L.

The molar volumes of other common gases at STP are H₂ =22.73 L; O₂ = 22.68 L; NH₃ = 22.56 L; H₂O = 22.50 L.

Simple answer, with some historical background are on this site: http://www.chemteam.info/GasLaw/MolarVolume.html

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