Molar Volume at STP: Why 22.4 L and 22.7 L Are Both Used
Almost every mole-concept question on gases says "at STP". Some books then use 22.4 L for one mole, and others use 22.7 L. Students lose marks when they mix the two or do not know why both exist. This article explains where each number comes from and how to choose between them.
What "molar volume" means
Molar volume is the volume occupied by one mole of a gas at a given temperature and pressure. For an ideal gas it comes from PV = nRT with n = 1:
It depends only on temperature and pressure. It does not depend on which gas it is. That is Avogadro's idea: equal volumes of ideal gases, at the same T and P, contain equal numbers of molecules.
Why two values: the definition of STP changed
"STP" means standard temperature and pressure. The temperature is 0 °C = 273.15 K in both definitions. The pressure is different:
| Definition | T | P | Molar volume of an ideal gas |
|---|---|---|---|
| Older definition | 273.15 K | 1 atm = 101.325 kPa | 22.41 L/mol (rounded to 22.4) |
| Current IUPAC definition | 273.15 K | 1 bar = 100 kPa | 22.71 L/mol (rounded to 22.7) |
Check the first value: Vm = 0.082057 L atm K−1 mol−1 × 273.15 K / 1 atm = 22.41 L/mol. Check the second: Vm = 0.083145 L bar K−1 mol−1 × 273.15 K / 1 bar = 22.71 L/mol. The gas has not changed. Only the standard pressure changed, and a slightly lower pressure gives a slightly larger volume.
There is a third number students meet: at 25 °C (298.15 K) and 1 bar, called SATP, the molar volume is 0.083145 × 298.15 / 1 = 24.79 L/mol. Do not use 22.4 or 22.7 for gases at room temperature.
Which value should you use?
Follow your textbook and your teacher. Many school books still use 22.4 L/mol at 0 °C and 1 atm, and some newer books give 22.7 L/mol at 0 °C and 1 bar. If the question states the pressure, use it: "1 atm" points to 22.4 and "1 bar" points to 22.7. If a question only says "STP", use the value your own course book uses, and write the value clearly in your answer so the examiner can follow your working. The difference is about 1.3 %, which is usually small, but it can change the last digit of the answer.
Using molar volume
Moles = volume ÷ Vm
Moles = mass ÷ molar mass
Step 1: Molar mass of CO2 = 12.01 + 2(16.00) = 44.01 g/mol.
Step 2: Moles = 11 / 44.01 = 0.2499 mol (about 0.25 mol).
Step 3: With 22.4 L/mol: 0.2499 × 22.4 = 5.60 L.
Step 4: With 22.7 L/mol: 0.2499 × 22.7 = 5.67 L.
Answer: 5.60 L (22.4 basis) or 5.67 L (22.7 basis). State which value you used.
Step 1: Moles = 5.6 / 22.4 = 0.25 mol.
Step 2: Mass = 0.25 × 32.00 = 8.0 g.
Answer: 8.0 g of O2.
Step 1: Moles of Zn = 6.5 / 65.38 = 0.0994 mol.
Step 2: The equation is 1 : 1, so moles of H2 = 0.0994 mol.
Step 3: Volume (22.4 basis) = 0.0994 × 22.4 = 2.23 L. Volume (22.7 basis) = 0.0994 × 22.7 = 2.26 L.
Answer: about 2.23 L (22.4) or 2.26 L (22.7).
Step 1: Density = molar mass / molar volume.
Step 2: With 22.4 L/mol: 44.01 / 22.4 = 1.965 g/L.
Step 3: With 22.7 L/mol: 44.01 / 22.7 = 1.939 g/L.
Answer: about 1.96 g/L or 1.94 g/L, depending on the STP definition.
Step 1: Moles = 5.6 / 22.4 = 0.25 mol.
Step 2: Molecules = 0.25 × 6.022 × 1023 = 1.51 × 1023.
Answer: about 1.5 × 1023 molecules. This does not depend on which gas it is, as long as it behaves ideally.
Common mistakes
- Using 22.4 L at room temperature. The value applies only at STP. At 25 °C and 1 bar the molar volume is about 24.8 L.
- Applying molar volume to liquids or solids. The idea of one molar volume for all substances works only for gases (ideally).
- Mixing the two values in one problem. If you use 22.4 for one step, do not use 22.7 in the next step of the same question.
- Forgetting the mole ratio. Convert gas volume to moles first, then use the balanced equation, then convert back.
- Forgetting units. Molar volume is L/mol. If your volume is in mL or dm³, convert carefully: 1 dm³ = 1 L = 1000 mL.
- Treating real gases as exactly ideal. The 22.4 and 22.7 figures are for an ideal gas. Real gases differ slightly.
Quick reference for exams
| Condition | Temperature | Pressure | Vm (L/mol) |
|---|---|---|---|
| Old STP (many school books) | 273.15 K | 1 atm | 22.41 |
| IUPAC STP (current) | 273.15 K | 1 bar | 22.71 |
| SATP / room conditions | 298.15 K | 1 bar | 24.79 |
For any other temperature and pressure, do not use a fixed number. Use V = nRT/P with the correct value of R for your units. Always read the question for the pressure unit before you choose a value.
Check any gas volume, pressure or temperature problem with PV = nRT after you have worked it by hand.
Open the Ideal Gas Law Calculator →Preparing for Class 11–12 chemistry? ABC Chemistry runs Class 11–12 coaching online, and online classes for students across India.