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Ideal-Gas Sample Molar Mass

Estimate a gas sample’s molar mass from mass, absolute pressure, volume and temperature using the ideal-gas law.

  • Formula & worked example
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Calculator inputs

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How to use this calculator

  1. Enter the known values in the units shown. Results update as you type.
  2. Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
  3. Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

At 100 kPa and 26.85°C, an ideal gas occupying 24.94338785445972 L contains 1 mol. A sample mass of 28 g therefore gives 28 g/mol.

Amount = pressure × volume / (R × absolute temperature); molar mass = mass / amount

Worked example

Enter these known values and leave the other values blank.

Absolute gas pressure
100 kPa
Gas temperature
26.85 °C
Gas sample volume
24.94338785445972 L
Gas sample mass
28 g
Amount of gas
1 mol
Estimated gas molar mass
28 g/mol

Assumptions and limitations

  • Pressure, volume, mass, amount and absolute temperature are positive. Enter absolute pressure, not gauge pressure; temperature selectors convert to kelvins internally.
  • The ideal-gas approximation uses R = 8.31446261815324 J/(mol·K). Real-gas effects, condensation and adsorption are excluded.
  • For a mixture, the result is the sample’s average molar mass when the ideal model is appropriate. All measurements must describe the same sample at the same state.

Common questions

Can I enter gauge pressure?

Convert gauge pressure to absolute pressure first by adding the applicable surrounding pressure. The gas law uses absolute pressure.

Is the molar volume always 22.4 liters?

No. Ideal molar volume depends on pressure and absolute temperature. Use the conditions of your actual sample.

References

Pounds-force per square inch use the exact international pound and inch with standard gravity, replacing the rounded captured factor. Ksi uses exactly one thousand of the same reviewed pounds-force per square inch. One technical atmosphere is exactly one kilogram-force per square centimeter under standard gravity. Bookify replaces the captured rounded US gallon with the exact 3.785411784-liter definition.

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