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Generated on: July 20, 2026 at 09:33 PM

Molar Volume of a Gas Calculator

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Amount of gas1 mol
Temperature273.2 K
Pressure101.325 kPa

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Chemistry

Molar Volume of a Gas Calculator

Find the volume an ideal gas occupies from its amount, temperature, and pressure using V = nRT/P. Returns both the total volume and the molar volume (L/mol), with STP presets that reproduce the familiar 22.4 L/mol.

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\begin{aligned} V &= n \cdot V_m = n \cdot \dfrac{RT}{P} \\ &= \left(1\,\text{mol}\right) \cdot ?\,\text{L} \\ &= ?\,\text{L} \end{aligned}

Details

\begin{aligned} V_m &= \dfrac{RT}{P} \\ &= \dfrac{0.0820573 \cdot T}{P} \\ &= ?\,\text{L} \end{aligned}

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What molar volume means

The molar volume of a gas is the space that exactly one mole of it occupies under a given temperature and pressure. A remarkable consequence of the ideal gas law is that this volume is the same for every gas — oxygen, hydrogen, carbon dioxide — provided the gas behaves ideally. Equal numbers of gas molecules take up equal volumes at the same conditions, which is Avogadro's law stated in modern terms.

The formula

Start from the ideal gas law, PV = nRT, and solve for volume:

V=nRTPV = \frac{nRT}{P}

Setting the amount to one mole (n = 1) gives the molar volume:

Vm=RTPV_m = \frac{RT}{P}

Here R is the universal gas constant, T is the absolute temperature, and P is the pressure. Using R = 0.082057 L·atm·mol⁻¹·K⁻¹ keeps the volume in litres when pressure is in atmospheres and temperature in kelvin.

SymbolQuantityUnit
VTotal volumeL
VₘMolar volumeL/mol
nAmount of gasmol
TAbsolute temperatureK
PPressureatm (or kPa, bar, ...)
RGas constant0.082057 L·atm·mol⁻¹·K⁻¹

Worked example

Find the volume of 2 mol of an ideal gas at 0 °C and 1 atm.

  1. Convert temperature to kelvin: 0 °C = 273.15 K.
  2. Molar volume: Vₘ = (0.082057 × 273.15) / 1 = 22.41 L/mol.
  3. Total volume: V = n × Vₘ = 2 × 22.41 = 44.83 L.

So two moles of any ideal gas at standard temperature and pressure occupy about 44.83 L.

The two STP conventions

Standard temperature and pressure (STP) has two definitions, and they give slightly different molar volumes. Quoting the wrong one is a common source of error in homework and lab reports.

ConventionTemperaturePressureMolar volume
Older STP0 °C1 atm (101.325 kPa)22.41 L/mol
IUPAC STP (since 1982)0 °C100 kPa22.71 L/mol
SATP (ambient)25 °C100 kPa24.79 L/mol

The classic "22.4 litres per mole" memorised in introductory chemistry refers to the older 1 atm convention. If a data table or exam specifies 100 kPa, use 22.71 L/mol instead.

When the ideal-gas assumption breaks down

The molar volume formula treats gas molecules as point particles with no mutual attraction. This works well at low pressure and high temperature, where molecules are far apart. Near a gas's boiling point, or at high pressure, intermolecular forces and finite molecular size cause measurable deviations. For those conditions a real-gas model such as the van der Waals equation gives a closer estimate.

Once the volume is known, the amount of gas links directly to mass through the molar mass, and to pressure or temperature changes through the combined gas law. The molar volume is also the bridge in gas stoichiometry: converting between moles of a gaseous reactant and the litres you measure in the lab.

Frequently Asked Questions (FAQ)

What is the molar volume of a gas?

The molar volume is the volume occupied by one mole of a gas at a stated temperature and pressure. For an ideal gas it is Vm = RT/P and depends only on the conditions, not on which gas it is. At 0 °C and 1 atm it is about 22.41 L/mol, and at 0 °C and 100 kPa it is about 22.71 L/mol.

Where does 22.4 L/mol come from?

Putting T = 273.15 K (0 °C) and P = 1 atm into Vm = RT/P with R = 0.082057 L·atm·mol⁻¹·K⁻¹ gives 22.41 L/mol. This is the classic figure quoted in textbooks for STP. It assumes ideal-gas behaviour, so real gases deviate slightly, especially near condensation.

Which STP definition should I use?

There are two. The older convention is 0 °C and 1 atm (101.325 kPa), giving 22.41 L/mol. Since 1982 IUPAC has defined STP as 0 °C and exactly 100 kPa, giving 22.71 L/mol. Check which one your course or data table assumes; the calculator handles both — just set the pressure accordingly.

How does molar volume change with temperature and pressure?

Because Vm = RT/P, the molar volume rises in direct proportion to absolute temperature and falls in inverse proportion to pressure. At 25 °C (298.15 K) and 1 atm it is about 24.47 L/mol, larger than the 0 °C value. Doubling the pressure halves the molar volume.

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