Rate of Effusion Calculator
Compare gas effusion rates from molar masses using Graham’s law.
01 Set up your calculation
- Enter the molar masses of the two gases.
- Enter a measured reference rate for gas B, or leave one to compare relative rates.
- Compare rates for gases under the same temperature and comparable effusion conditions.
02 The relationship
For the assumptions behind these results, read the explanation below. You can also explore Molar Mass and Molar Mass of Gas.
Follow the numbers
For gases with molar masses 4 and 16 g/mol, rA/rB = √(16/4) = 2. Gas A effuses twice as fast and needs half the time for the same amount.
03 Why lighter particles have a larger rate
At the same temperature, lighter gas particles have greater characteristic molecular speeds than heavier particles. Under the assumptions of effusion through a very small opening, the relative escape rate follows the inverse square root of molar mass.
The ratio is dimensionless. If you supply a reference rate in moles per minute, the predicted rate uses moles per minute; the tool does not infer a unit from the number alone.
04 Distinguish effusion from bulk mixing
Effusion concerns escape through an opening small enough for individual molecular motion to control passage. Diffusion through another gas also depends on collisions, concentration gradients, and geometry. A room-scale mixing time cannot be predicted from the molar-mass ratio alone.
Keep temperature, pressure, and apparatus conditions comparable. A bulk leak, pressure-driven flow, or changing gas mixture may require a different transport model.
Questions from the notebook
Why is the molar-mass ratio reversed?
Rate increases as molar mass decreases, so MA appears in the denominator when calculating rA/rB.
Can this give an absolute rate without a reference?
No. It gives a relative rate; apparatus and conditions set the absolute scale.
Can I compare effusion times?
Yes. For equal gas amounts under matching conditions, the time ratio is the reciprocal of the rate ratio.
References & reading
- OpenStax, Rice University — Effusion and Diffusion of Gases ↗
States Graham’s inverse-square-root relationship for molar masses and effusion rates.
- NIST — Guide to SI conversion factors ↗
Supports physical unit conversions. Input values are converted to the formula’s base units before calculation.
Our explanations are written for ChemistryCalc. Read how we build and check our tools. These calculations are educational estimates where the underlying model requires assumptions.