Gas diffusion and relative molecular mass
Compare gases at the same temperature without treating mass as a change in particle size.
Exam scope and assessment notes
Extended only (Supplement) · Chemistry 0971, 2026–2028. Cambridge states that IGCSE 0971 has the same subject content as IGCSE 0620, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Explain diffusion using random particle movement.
- Read a chemical formula to count its atoms.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What is the relative molecular mass of H₂ using H=1?
What you will learn
- Predict which gas diffuses faster from relative molecular masses.
- Explain the comparison using average particle speeds at the same temperature.
Keep temperature the same
At the same temperature, different gases have the same average kinetic energy, but their particles need not have the same average speed. A lighter particle moves faster on average than a heavier one at that temperature. Gases with lower relative molecular mass generally diffuse faster under comparable conditions.
Compare the relative mass of the complete molecule. Hydrogen is H₂, so its relative molecular mass is 2 using H=1; oxygen is O₂, so its value is 32 using O=16. The comparison is about mass and speed, not the size of spaces inside the molecule.
Use evidence with controlled conditions
If two gases begin at opposite ends of a tube and form a visible product where they meet, the faster-diffusing gas usually travels farther before meeting the other. Under the familiar idealised comparison, the meeting point lies nearer the source of the slower gas. Unequal temperatures or strong bulk airflow would complicate this interpretation.
For this syllabus, make a qualitative prediction and explain it. A claim that twice the molecular mass means exactly half the speed is not justified by the simple comparison. A heavier gas does not fail to diffuse: its particles still move randomly.
Pause and explain
Why does a lighter gas generally diffuse faster at the same temperature?
Worked example
At the same temperature, compare the diffusion of methane, CH₄, and carbon dioxide, CO₂. Use C=12, H=1 and O=16.
Show the worked solution
- CH₄ has relative molecular mass 12+4=16.
- CO₂ has relative molecular mass 12+32=44.
- Methane particles have the lower mass and higher average speed at that temperature.
Answer Methane diffuses faster under comparable conditions.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Compare hydrogen (Mᵣ=2) and oxygen (Mᵣ=32) at the same temperature.
Give me a hint
Identify the lighter molecule before predicting speed.
Compare my reasoning
- Hydrogen has lower relative molecular mass.
- Its molecules have higher average speed at the same temperature.
- Hydrogen therefore diffuses faster under comparable conditions.
Hydrogen diffuses faster because its lighter molecules move faster on average.
Look for these in your work
- The complete molecular masses are compared.
- Speed links molecular mass to diffusion.
Which is faster under matching conditions: helium, relative mass 4, or nitrogen, Mᵣ=28? Explain.
Give me a hint
Helium is monatomic; compare the masses of the actual gas particles.
Compare my reasoning
- Helium particles have relative mass 4.
- Nitrogen molecules have relative mass 28.
- The lighter helium particles move faster on average at the same temperature.
Helium diffuses faster under matching conditions.
Look for these in your work
- The gaseous particles' masses are compared.
- The prediction includes comparable temperature and conditions.
In an idealised still tube, gases X (Mᵣ=20) and Y (Mᵣ=40) enter at opposite ends and react where they meet. Which source is the meeting point nearer, and what condition could spoil the comparison?
Give me a hint
The faster gas travels farther; consider bulk movement as a competing process.
Compare my reasoning
- X is lighter and diffuses faster at the same temperature.
- X travels farther, so the meeting point is nearer Y's source.
- Strong airflow or different temperatures would complicate the prediction.
Nearer Y's source; airflow or unequal temperatures could invalidate the simple comparison.
Look for these in your work
- The meeting position follows the correct speed comparison.
- A relevant experimental limitation is identified.
Common mistakes
- Comparing the atomic mass of only one element instead of the entire molecule.
- Claiming an exact speed ratio without a supported relationship.
What can you explain now?
Which diffuses faster under comparable conditions at one temperature: NH₃ (Mᵣ=17) or HCl (Mᵣ=36.5)?
Compare with the explanation
NH₃
The lighter ammonia molecules move faster on average at the same temperature.
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Make it stick
Choose two familiar molecular formulas, calculate their relative masses and predict their diffusion order at the same temperature without inventing an exact ratio.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.