Cambridge IGCSE Chemistry · Syllabus 0620 · States of Matter
Relative molecular mass
What is Relative molecular mass?
A number, written Mr, defined as the sum of the relative atomic masses of the atoms shown in the formula of a substance. It is found by adding together the relative atomic mass of every atom in that formula, and it has no units because each relative atomic mass is itself a comparison of one mass with another. In the study of diffusion it is used to compare gases: at the same temperature, a gas with a lower relative molecular mass diffuses faster than a gas with a higher relative molecular mass, so it travels further in the same time.
This definition is part of the States of Matter chapter in Cambridge IGCSE Chemistry.
Common mistakes with Relative molecular mass
- 6. “The heavier gas sinks to the bottom and the lighter gas floats to the top.” Repair Gas particles move rapidly and randomly in all directions, so two gases released into the same space spread through one another and mix — a higher relative molecular mass makes a gas diffuse more slowly, not downwards. Taught in Lesson F Core 1.2.1 and Lesson G Supplement 1.2.2
Examiner tips on Relative molecular mass
- This is the exception in Topic 1, so read the statement carefully. Everywhere else in this chapter, Core describes and Supplement explains. Statement 1.2.1 asks every candidate to describe and explain diffusion in terms of kinetic particle theory, so the whole particle mechanism in this lesson — random motion, net movement, the counting argument — is Core material and can be asked on the Core papers. Only the effect of relative molecular mass on the rate of diffusion is Supplement, and that is the separate statement 1.2.2 in Lesson G.
- Two words that must be in the answer. Say relative molecular mass, not “it is lighter” or “it is less dense”. And say at the same temperature, because a hotter heavy gas can easily out-diffuse a colder light one — the comparison only holds when the temperature is the same for both.