One mole of any ideal gas takes the same volume — it comes straight from PV = nRT
The molar volume is just the ideal gas law solved for volume per mole. Because it does not depend on the identity of the gas, one mole of helium, nitrogen or CO₂ all occupy the same space at the same temperature and pressure. To get the volume of any amount, multiply: V = n × Vm.
Which "standard" you mean changes the number
There is no single STP. The classic 22.4 value uses 1 atm; IUPAC moved the standard to 1 bar in 1982, and the ambient version (SATP) uses room temperature. Same gas law, different inputs.
| Standard | Temperature | Pressure | Molar volume |
|---|---|---|---|
| Classic STP | 0°C (273.15 K) | 1 atm (101.325 kPa) | 22.414 L/mol |
| IUPAC STP (1982+) | 0°C (273.15 K) | 1 bar (100 kPa) | 22.711 L/mol |
| SATP | 25°C (298.15 K) | 1 bar (100 kPa) | 24.790 L/mol |
| NTP (US) | 20°C (293.15 K) | 1 atm (101.325 kPa) | 24.055 L/mol |
Older textbooks and exams still use classic STP (22.414). The current IUPAC standard is 22.711, because 1 bar is slightly below 1 atm so the gas expands a little more.
Why 22.4 and 22.7 differ — it is only the pressure
Both are 0°C. Dropping the pressure from 1 atm to the slightly lower 1 bar lets the same mole spread out by about 1.3% — the whole gap between the two "standard" numbers.
The mistakes that throw the answer off
- Celsius instead of kelvin. R needs absolute temperature. Add 273.15 to any Celsius value before it enters the formula.
- Mismatched units. Use R = 8.314 J/(mol·K) with pascals and cubic metres, or R = 0.08206 L·atm/(mol·K) with atm and litres — never mix the two sets.
- Assuming ideal behaviour always holds. PV = nRT is within about 1% for common gases near room conditions, but drifts off under high pressure or near condensation.
Common questions
Is the molar volume at STP 22.4 or 22.7 litres?
Both are correct for different definitions of STP. The 22.414 L/mol figure taught in most textbooks uses 0 degrees C and 1 atm. IUPAC redefined STP in 1982 to use 1 bar instead of 1 atm, which gives 22.711 L/mol. Check which pressure your problem assumes.
Why must temperature be in kelvin for the ideal gas law?
The gas constant R carries units of joules per mole per kelvin, so PV = nRT only balances when T is an absolute temperature. Plugging in 25 for 25 degrees C instead of 298.15 K gives an answer roughly 12 times too small.
How do I find the volume of a gas at STP?
Multiply the number of moles by the molar volume for your chosen standard. At classic STP, 2 moles of any ideal gas occupy 2 x 22.414 = 44.83 L, regardless of which gas it is.
