Chemistry

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Graham's Law Calculator.

Calculate relative effusion rate from two molar masses.

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Relative rate: 4

Relative rate

4.000000

Educational chemistry arithmetic only. Keep units consistent and verify assumptions against your laboratory or course specification.

Use Cases

Compare effusion rates of two gases

Use this calculator to quickly compare how fast one gas effuses relative to another. This is useful in chemistry labs or studies involving gas separation or diffusion.

Example: Compare O2 (32 g/mol) and H2 (2 g/mol): relative rate = sqrt(32/2) = 4, so H2 effuses 4 times faster.

Estimate molar mass from effusion data

If you know the relative effusion rate and one molar mass, you can rearrange Graham's Law to find the unknown molar mass. This calculator helps you verify the rate part of the equation.

Example: If rate ratio is 2 and M1 = 4 g/mol, then M2 = (rate ratio)^2 * M1 = 4 * 4 = 16 g/mol.

Frequently Asked Questions

What is Graham's Law and how does this calculator use it?
Graham's Law states that the rate of effusion of a gas is inversely proportional to the square root of its molar mass. This calculator uses the molar masses you enter for Gas 1 and Gas 2 to compute the relative effusion rate (rate1/rate2) as sqrt(M2/M1).
What units should I use for molar mass?
Enter the molar masses in grams per mole (g/mol). The calculator expects values in g/mol for both Gas 1 and Gas 2. Ensure you use the correct molar mass for each gas (e.g., O2 = 32 g/mol, H2 = 2 g/mol).
Can this calculator determine the actual effusion time or rate?
No, this calculator only computes the relative effusion rate between two gases based on their molar masses. It does not provide absolute rates or times, which would require additional conditions like temperature and pressure.

Tips & Common Mistakes

Tips

  • Ensure molar masses are entered in g/mol; use the periodic table for accurate values.
  • Remember that the relative effusion rate is inversely proportional to the square root of molar mass: lighter gases effuse faster.
  • For a quick check, if both molar masses are equal, the relative rate is 1, meaning equal effusion rates.
  • Use this calculator for educational purposes to verify your manual calculations.

Common Mistakes to Avoid

  • Using atomic mass instead of molar mass for diatomic gases (e.g., using 16 for O instead of 32 for O2).
  • Reversing the order of molar masses, which would give the inverse rate ratio.
  • Forgetting to take the square root when calculating the rate ratio.

Last updated: August 13, 2026