Chemistry / Unit 7 Chem 1254 worked example, 60 to 90 seconds
Standard Molar Volume, Computed Under Both STP Conventions
The idea
Computing the standard molar volume means substituting one mole and 273.15 kelvin into the ideal gas law and solving for volume, once under each STP pressure convention.
In practice
Using P = 1 atm and R = 0.08206 L·atm/mol·K: V = nRT/P = (1)(0.08206)(273.15)/1 ≈ 22.4 L. Using P = 1 bar (0.9869 atm) instead, the same substitution gives V = (1)(0.08206)(273.15)/0.9869 ≈ 22.7 L. The two conventions' standard molar volumes differ only because their pressure definitions differ, not because of any change in the gas or the temperature.
Where people slip
Treating 22.4 L/mol as the universal standard molar volume, without checking which pressure convention set up the calculation, silently picks the wrong one for a current-IUPAC-sourced problem — and the reverse error, reading 22.7 L/mol into an older, 1-atm-based source, is equally easy to make.
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