A Level · Cambridge (CIE) · 9701

Moles and molar mass

Use the mole to connect a measured mass with a chemical amount.

What you will learn

Calculate amount from mass.
Distinguish molar mass from relative formula mass.

Amount counts entities

The mole is a unit of amount of substance. One mole contains Avogadro's number of specified entities. The entity must be clear: atoms, molecules, ions or formula units are not interchangeable descriptions.

Molar mass is mass per mole, often in g mol⁻¹. Relative formula mass is dimensionless; its numerical value corresponds to molar mass in g mol⁻¹ at the precision used in these calculations.

Use units to check rearrangement

Dividing grams by grams per mole gives moles. If a chemical equation is involved, first calculate the amount of the known reactant, then use the balanced mole ratio, and only afterwards convert to the requested mass or other quantity.

n=m/M

Put the idea to work

Worked example

A sample has mass 9.0 g and molar mass 60.0 g mol⁻¹. Find its amount.

Show the worked solution

Use n=m/M.
Substitute 9.0/60.0.
Report the amount with the unit mol.

Answer 0.15 mol

Common mistakes

Do not multiply mass by molar mass.
A balanced equation's coefficients give mole ratios, not generally mass ratios.

Recall without your notes

What can you explain now?

What mass is 0.25 mol of a substance with M=80 g mol⁻¹?

Compare with the explanation

20 g

Rearrange to m=nM=0.25×80.

After trying it yourself, choose your next review. This is your self-assessment.

Your review choice appears on Today. Sign in to sync it across devices.

Practise this idea Continue to next lesson Back to Today

Keep learning Chemistry

Check the official syllabus 2028–2030

Course scope and assessment

Four hundred and forty-two original lessons map the published learning outcomes across AS Chemistry Topics 1–22 and A Level Topics 23–37, with guided, independent and transfer practice and explained answers. Every numbered subject-content topic and listed outcome tag now has a mapped lesson sequence. Topic 32 maps all 12 hydroxy-compound outcome tags in 11 lessons; Topic 33 maps all 15 carboxylic-acid and derivative outcome tags in 16 lessons; Topic 34 maps all 22 nitrogen-compound outcome tags in 20 lessons; Topic 35 maps all 12 polymerisation outcome tags in 16 lessons; Topic 36 maps all four organic-synthesis outcome tags in 14 lessons; Topic 37 maps all 22 chromatography and NMR outcome tags in 18 lessons. Cambridge's 2028–2030 Version 1 syllabus states that there are no significant changes which affect teaching from the preceding edition; the routes are aligned to that edition. This is 37 of the 37 numbered subject-content topics. Full cumulative exam assessment, Paper 3 laboratory competence, Paper 5 investigation practice and human subject-teacher review remain outstanding. Written practice is self-checked and does not certify exam readiness.