Compare effusion or diffusion rates of two gases from their molar masses.
| r1 / r2 | – |
Use this calculator for a pinhole demonstration or homework: helium versus nitrogen, or which gas leaves a balloon first.
The calculation is rate₁ / rate₂ = √(M₂ / M₁). The lighter gas is faster. Both gases are assumed to be at the same temperature and to pass the same small orifice.
This is Graham’s law for effusion (and an approximation for diffusion). It does not include viscosity, turbulence, or a mean-free-path correction.
M of gas 1: Molar mass of gas 1 in g/mol. Helium is about 4.
M of gas 2: Molar mass of gas 2 in g/mol. Nitrogen is about 28.
r1 / r2: Rate of gas 1 divided by the rate of gas 2. A value of 2.65 means gas 1 effuses about 2.65 times as fast as gas 2.
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