STP and Molar Volume: Why Is It 22.4 L (or 22.7 L)?

Ask three chemistry textbooks for the volume of one mole of gas at STP and you might get three answers: 22.4 L, 22.7 L or even 24.5 L. None of them is a mistake. They’re using different definitions of “standard” conditions, and knowing which is which saves a lot of confusion.

What “STP” is for

The volume of a gas depends strongly on its temperature and pressure (see the gas laws). To compare gases fairly, or to quote a single number for “the volume of a mole of gas”, scientists need agreed reference conditions. STP — standard temperature and pressure — is that agreement.

The two main definitions

The older definition: 0 °C and 1 atm

For most of the 20th century, STP meant 273.15 K (0 °C) and 1 atmosphere (101.325 kPa). Under those conditions one mole of an ideal gas occupies:

V = nRT ÷ P = 1 × 0.082 057 × 273.15 ÷ 1 = 22.414 L

This is the famous 22.4 L that appears in many textbooks and exam formula sheets.

The current IUPAC definition: 0 °C and 100 kPa

In 1982 IUPAC changed standard pressure to exactly 100 kPa (1 bar), a rounder number in SI units. It’s only about 1.3% lower than 1 atm, but it changes the molar volume:

V = 1 × 8.314 46 × 273.15 ÷ 100 = 22.711 L

So strictly by IUPAC’s definition, the molar volume at STP is 22.7 L. Many courses still use 22.4 L, so check which one your syllabus or exam expects.

Room temperature: SATP and “RTP”

Nobody does experiments at 0 °C. For conditions closer to a real lab, you’ll see:

• SATP (standard ambient temperature and pressure): 25 °C (298.15 K) and 100 kPa → molar volume 24.79 L
• “RTP” in some school syllabuses: often 20 °C or 25 °C and 1 atm → around 24.0 L or 24.5 L

These are all just PV = nRT with different T and P plugged in.

Using molar volume

At a fixed temperature and pressure, every ideal gas has the same molar volume, whatever the gas. That’s Avogadro’s law: equal volumes of gases at the same conditions contain equal numbers of molecules. It makes gas calculations very quick:

moles of gas = volume ÷ molar volume

How many moles are in 5.6 L of oxygen at 0 °C and 1 atm? 5.6 ÷ 22.4 = 0.25 mol

What volume does 88 g of CO₂ occupy at 0 °C and 1 atm? 88 g ÷ 44.01 g/mol = 2.00 mol → 2.00 × 22.4 = 44.8 L

In a reaction, what volume of hydrogen reacts with 3.0 L of nitrogen to make ammonia (same conditions)? N₂ + 3H₂ → 2NH₃. Because volume is proportional to moles for gases at the same conditions, the ratio of volumes equals the ratio of coefficients: 3.0 L × 3 = 9.0 L of H₂. You don’t even need the molar volume.

When not to use it

The molar volume shortcut only works when:

• The substance is a gas under those conditions. Water at 0 °C is not a gas; 18 g of liquid water is 18 mL, not 22.4 L.
• The conditions are exactly the ones the molar volume was calculated for. At any other temperature or pressure, use PV = nRT directly.
• The gas behaves ideally. Real gases deviate by up to a few percent at these conditions — carbon dioxide’s real molar volume at 0 °C and 1 atm is about 22.26 L, for example.

Quick answers

Is 22.4 L wrong? No. It’s correct for 0 °C and 1 atm, the older definition of STP. It’s just not the current IUPAC standard.

Does the type of gas matter? Not for an ideal gas. A mole of helium and a mole of sulfur hexafluoride occupy essentially the same volume at the same conditions, even though the second weighs 36 times more.

What is “standard state” in thermodynamics? A related but different idea: it specifies a standard pressure (1 bar) but no fixed temperature. Tables of enthalpies are usually given at 298.15 K.

Calculate it

The ideal gas law calculator finds the volume of any amount of gas at any temperature and pressure — no need to remember which STP your textbook uses.