A-Level Chemistry 9701 / Topic 2 / AS

Counting by weighing.

Nobody can count 10²³ atoms, but anyone can weigh 12 grams of carbon. The mole links the two: a fixed number of particles whose mass in grams equals the relative mass. It is the unit every chemical calculation runs through.

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The key idea

One mole is the amount of substance that contains 6.02 × 10²³ particles, the Avogadro constant L. That number is chosen so that one mole of carbon-12 atoms has a mass of exactly 12 g, which makes the molar mass M of any substance in g mol⁻¹ numerically equal to its relative mass. So n = m ÷ M and N = n × L. Always say which particle you are counting: atoms, molecules, ions or formula units.

Fig. 1: the mole sits between mass and number of particles

Section 01

A chemist's dozen.

Pick a substance in the simulator and drag the mass slider. Watch the amount in moles and the number of particles change together. Try 12 g of carbon, 18 g of water and 58.5 g of sodium chloride and notice what they share.

Substance	Relative mass	Mass of 1 mol	Contains
carbon, C	12.0	12.0 g	6.02 × 10²³ atoms
water, H₂O	18.0	18.0 g	6.02 × 10²³ molecules
sodium chloride, NaCl	58.5	58.5 g	6.02 × 10²³ formula units

Section 02

Mass, moles, particles.

Work every conversion through the mole. Divide by M to go from grams to moles; multiply by L to go from moles to particles.

9.0 g of water: n = 9.0 ÷ 18.0 = 0.50 mol of molecules.
Molecules: 0.50 × 6.02 × 10²³ = 3.01 × 10²³ molecules of H₂O.
Atoms: each molecule has three atoms, so 3 × 3.01 × 10²³ = 9.03 × 10²³ atoms.
Mass of one molecule: 18.0 g ÷ 6.02 × 10²³ = 2.99 × 10⁻²³ g.

Stage 1 · Learn

Check what the sim just showed you

Four quick checks. Each correct answer earns XP and records this skill on your profile.

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The Avogadro constant is:

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How many moles are there in 20.0 g of sodium hydroxide, NaOH (M_r 40.0)?

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The units of molar mass are:

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How many atoms are there in 1.00 mol of ammonia, NH₃?

Section 03

Which particle?

The word mole means nothing until you say what it is a mole of. Most lost marks in this section come from counting the wrong thing.

Elements that are molecules: 32.0 g of oxygen is 1 mol of O₂ molecules but 2 mol of O atoms.
Ionic compounds: 1 mol of CaCl₂ contains 1 mol of Ca²⁺ ions and 2 mol of Cl⁻ ions, 3 mol of ions in total.
Units: molar mass is in g mol⁻¹; L is in mol⁻¹. Relative masses themselves have no units.

Examiner trap

When a question asks for the number of atoms or ions, multiply the number of formula units by how many of that particle each one contains. 0.100 mol of MgCl₂ contains 0.200 mol of chloride ions, not 0.100 mol.

Stage 2 · Exam

Exam-style questions

Unlocks once the four checks above are done. Worth more XP, written to AS Paper 1 and Paper 2 standard.

Finish the four checks above to unlock the exam questions

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What is the mass of 0.250 mol of sodium carbonate, Na₂CO₃? (A_r: C 12.0, O 16.0, Na 23.0)

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How many chloride ions are present in 0.100 mol of magnesium chloride, MgCl₂? (L = 6.02 × 10²³ mol⁻¹)

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Which sample contains the greatest number of atoms?

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What is the mass of one atom of carbon-12? (L = 6.02 × 10²³ mol⁻¹)

Skill unlocked

The mole, mastered.

This skill is now recorded on your profile. Keep the chain going.

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Go deeper · practice

Six original Cambridge-style questions

Defining the mole, converting between mass, moles and particles, counting atoms and ions, deducing M_r and identifying an element from a molar mass.