Standard Conditions of Temperature and Pressure

We have seen that the volume of a given quantity of gas and the number of molecules (moles) in a given volume of gas vary with changes in pressure and temperature. Chemists sometimes make comparisons against a standard temperature and pressure (STP) for reporting properties of gases: 273.15 K and 1 atm (101.325 kPa).[footnote] At STP, one mole of an ideal gas has a volume of about 22.4 L—this is referred to as the standard molar volume ([link]). The IUPAC definition of standard pressure was changed from 1 atm to 1 bar (100 kPa) in 1982, but the prior definition remains in use by many literature resources and will be used in this text.

Regardless of its chemical identity, one mole of gas behaving ideally occupies a volume of ~22.4 L at STP.