Chemistry / Chem 1254 · Procedure · 60–90 seconds

Standard Molar Volume, Computed Under Both STP Conventions

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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.

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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.

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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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