IB Chemistry SLTopic 1 — Counting Particles by MassPaper 1 & 2Core skill~9 min read
Molar Mass
You can’t count atoms directly, but you can weigh them. Molar mass is the bridge: it links the mass you measure on a balance to the number of moles — and from there to the actual number of particles.
📘 What you need to know
The molar mass (M) of a substance is its Ar or Mr expressed in grams, with units g mol-1.
moles = mass ÷ molar mass (a formula triangle links moles, mass and M).
number of particles = moles × Avogadro constant (L).
Watch molecule composition: 1 mole of a molecule contains several moles of atoms.
Why we use moles
Atoms are so tiny that even a pinch of a substance contains an astronomical number of them. Those numbers are impossible to work with directly, so we bundle particles into moles — a practical counting unit. Two simple formula triangles handle all the conversions.
Linking moles and particles
To go between a number of moles and the actual number of particles, use the Avogadro constant (L):
Cover the quantity you want: particles = moles × L, and moles = particles ÷ L. (L = 6.02 × 10²³ mol⁻¹.)
WORKED EXAMPLE
How many hydrogen atoms are in 0.010 moles of CH3CHO?
Count H atoms per moleculeCH₃CHO has 4 H atoms, so 0.010 mol of molecules = 0.040 mol of H atoms.atoms = moles × L0.040 × (6.02 × 10²³)= 2.4 × 10²² H atoms
Linking moles and mass
The molar mass (M) is the relative mass in grams — units g mol-1. A second formula triangle links moles, mass and molar mass:
Cover the quantity you want: mass = moles × M, and moles = mass ÷ M. (M in g mol⁻¹.)
The key relationship
moles = mass (g) ÷ molar mass (g mol-1)
WORKED EXAMPLE
(a) What is the mass of 0.250 moles of zinc (Ar = 65.38)? (b) How many moles are in 2.64 g of sucrose, C12H11O22 (Mr = 342.3)?
(a) mass = moles × M0.250 × 65.38 = 16.3 g(b) moles = mass ÷ M2.64 ÷ 342.3 = 7.71 × 10⁻³ mol16.3 g · 7.71 × 10⁻³ mol
💡 Exam tip
Always show your working — it’s easier to spot slips, and you can pick up method marks even if the final number is wrong.
Check units before you start: mass in grams, molar mass in g mol-1.
Up next: Empirical Formulae — working out the simplest whole-number ratio of atoms in a compound from mass or percentage data.
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