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1.4.1Additional lessons

The Mole Unit — IB Chemistry SL Revision Notes & Topical Questions

5 min read 5 parts Lesson 11 of 98

📚 IB Chemistry SL Hub / Topical Revision
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Part 1 of 5The Mole

1.4.1 The Mole Unit

The mole lets chemists count extremely large numbers of tiny particles.

Definition
  • One mole is the amount of substance containing the same number of elementary entities as there are atoms in 12 g of carbon-12.
Avogadro constant
  • The number of particles in one mole is 6.02 × 10²³ mol⁻¹.
Elementary entities
  • The particles may be atoms, molecules, ions, electrons or formula units, so the entity should always be stated.
Why use it
  • Amounts of reacting substances are compared in moles because balanced equations relate numbers of particles.

Part 2 of 5Using the Relationship

Number of particles N and amount n in mol are linked by the Avogadro constant L.

1
The equation is N = n × L, where L = 6.02 × 10²³ mol⁻¹.
2
To find the amount from a number of particles, rearrange it to n = N ÷ L.
3
To count atoms in a molecule, first find the number of molecules and then multiply by the atoms in one molecule.
4
To count ions in a formula unit, multiply the number of formula units by the number of ions each one releases.

Part 3 of 5Common Traps

Common Traps
Which particle
  • One mole of O₂ contains 6.02 × 10²³ molecules but twice as many oxygen atoms.
Standard form
  • Write large answers in standard form with a suitable number of significant figures.
Units
  • The unit of amount is the mole, symbol mol, and the Avogadro constant has units of mol⁻¹.
Mass link
  • One mole of carbon-12 atoms has a mass of exactly 12 g, which connects the mole to measurable mass.

Part 4 of 5Worked Example

Question How many molecules and how many atoms are in 0.250 mol of water, H₂O?

  1. Molecules N = n × L = 0.250 × 6.02 × 10²³ = 1.505 × 10²³.
  2. Each molecule has 3 atoms: 1.505 × 10²³ × 3 = 4.515 × 10²³.

Answer 1.51 × 10²³ molecules and 4.52 × 10²³ atoms

Part 5 of 5Exam Checklist & Tips

1.4.1 – Key Points for the Exam

  • Mole The amount of substance that contains 6.02 × 10²³ elementary entities.
  • Avogadro constant The number of particles in one mole, 6.02 × 10²³ mol⁻¹.
  • Amount of substance The quantity measured in moles, symbol n.
  • Formula unit The simplest ratio of ions in an ionic compound.

Common Mistakes

  • Treating the mole as a mass instead of an amount of substance.
  • Forgetting to say whether molecules or atoms are being counted.
  • Dividing instead of multiplying when converting moles to number of particles.
  • Writing the Avogadro constant without the unit mol⁻¹.

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