Twenty-two point four litres per mole is one of the few numbers that survives chemistry class intact. It gets memorised, it gets used, and it gets written on whiteboards for decades afterwards with three words attached: "at STP".
Those three words are where it goes wrong. Not the number — the number is fine. STP is not one set of conditions, and the same mole of gas occupies 22.414 litres or 22.711 litres depending on which one you meant.
Two standards, one abbreviation
The molar volume of an ideal gas is RT/p, so it moves with whatever pressure you call standard. Both candidates are published, both are exact, and NIST lists them side by side as separate named constants:
molar volume of ideal gas (273.15 K, 100 kPa) = 22.710 954 64 × 10⁻³ m³/mol
molar volume of ideal gas (273.15 K, 101.325 kPa) = 22.413 969 54 × 10⁻³ m³/mol
The second is the familiar 22.4, computed at one atmosphere. The first is computed at one bar, and the difference between them is 1.3% — small enough to pass unnoticed in a rough calculation, large enough to matter in a graded one.
What changed in 1982, and what did not
IUPAC's Green Book records the change in a single sentence:
"The value for p⦵ = 100 kPa, is the IUPAC recommendation since 1982, and is recommended for tabulating thermodynamic data."
⚠️ Read the scope of that carefully, because it is narrower than it is usually reported. The 1982 recommendation is about standard pressure for tabulating thermodynamic data — standard enthalpies, standard entropies. It is not worded as a redefinition of the teaching abbreviation STP, and IUPAC did not declare 22.4 an error. The same Green Book still defines the amagat using one atmosphere and prints the molar volume there as roughly 22.4 × 10⁻³ m³/mol.
So "22.4 has been wrong since 1982" is too strong. What happened is that the convention it comes from was superseded for one important purpose, and the abbreviation kept being taught as though there were only ever one.
The exam boards do not agree either
This is the part that makes the problem concrete, because it means a student can be marked wrong for the right arithmetic.
The International Baccalaureate moved. Its data booklet gives the molar volume of an ideal gas at STP as 22.7 dm³ mol⁻¹, and states its STP as 273 K and 100 kPa.
OCR's A Level chemistry data sheet does not use the term at all — it supplies "Molar gas volume = 24.0 dm³ mol⁻¹ at room temperature and pressure", a third convention entirely. AQA supplies the figure in the question stem when it is needed rather than in the booklet.
The Chinese curriculum teaches 22.4 L/mol and states its conditions: 标准状况, 0 °C and 101 kPa. That is internally consistent and correct for the syllabus that defines it.
⚠️ Four authorities, four answers — 22.4, 22.7, 24.0, and "we will tell you" — and every one of them is defensible, because each states its conditions. None of them is the failure mode.
What the failure mode actually is
The defect is not the value. It is the bare phrase "22.4 L at STP" with nothing attached, which is true under one convention, false under another, and gives the reader no way to tell which was meant.
A page that says "22.4 L at 0 °C and 1 atm" is correct forever. A page that says "22.4 L at STP" was correct in 1975 and is now ambiguous in a way that quietly transfers the error to whoever uses it.
One number that did become exact
A detail worth knowing if you are checking old figures against new ones. Both molar volumes above are marked exact, and they were not always.
Since the SI redefinition took effect in May 2019, the gas constant R is exact by construction: it is the Avogadro constant times the Boltzmann constant, and both of those are now defined values rather than measured ones. R is 8.314 462 618… J mol⁻¹ K⁻¹ with no uncertainty, so any molar volume derived from it at a stated temperature and pressure is exact too.
⚠️ Before 2019 the same constants were published with uncertainties — the 2007 Green Book printed 22.710 981(40) and 22.413 996(39), which differ from today's values in the fifth and sixth significant figures. So 22.711 is itself a post-2019 number, and an older source disagreeing slightly is not wrong, only older.
What this does not mean
It does not mean anyone teaching 22.4 is teaching an error. With the conditions stated it is exactly right, and for most school problems the choice of convention changes nothing that matters.
Nor does it mean IUPAC's 100 kPa is the one true standard. It is a recommendation, made for a stated purpose, and the organisation that made it still publishes the other value in its own documents.
What to do with it
Write the conditions next to the number, every time, in six words: 22.4 L at 0 °C and 1 atm. That single habit removes the entire ambiguity and costs nothing.
If you are checking a calculation against a textbook and the answers differ by about 1.3%, this is almost certainly the reason — one of you used 100 kPa and the other 101.325 kPa.
And if you are sitting an exam, use the data booklet's value rather than the one you memorised, because the booklet is the convention you are being marked against.
Where this comes from
The 1982 recommendation is quoted from IUPAC's "Quantities, Units and Symbols in Physical Chemistry", third edition, section 2.11.1, which cites J. D. Cox, Pure and Applied Chemistry volume 54, pages 1239 to 1250, 1982. The two molar volumes and the exactness of the gas constant are from the NIST reference on constants, units and uncertainty, CODATA 2022 values. Exam figures are from the IB chemistry data booklet and the OCR A Level chemistry data sheet. All were read on 16 September 2026.
⚠️ Both molar volumes were recomputed here from R, 273.15 K and each pressure rather than copied, and they agree with the published constants digit for digit at the precision quoted. One claim was dropped during checking: a widely repeated line that NIST defines standard conditions as 20 °C could not be traced to any NIST document, while searches on the same domain readily returned NIST primary material on neighbouring topics. It is not in this piece, because it could not be sourced.
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