The Mole
Section: 3. Stoichiometry | Syllabus: Cambridge IGCSE Chemistry (0620)
The Mole and the Avogadro Constant
The mole (mol) is the unit for the amount of substance. One mole of any substance contains exactly 6.02 × 10²³ particles - this number is called the Avogadro constant (NA).
- Like "a dozen" always means 12 items, "a mole" always means 6.02 × 10²³ particles, whatever the particle is
- 1 mole of carbon atoms = 6.02 × 10²³ carbon atoms
- 1 mole of water molecules = 6.02 × 10²³ H₂O molecules
- 1 mole of sodium ions = 6.02 × 10²³ Na⁺ ions
Individual atoms and molecules are far too small to count or weigh one at a time. The mole gives us a practical-sized "counting unit" that links the microscopic world of particles to masses we can actually measure on a balance.
Just as "a dozen" always means 12 items, "a mole" always means 6.02 × 10²³ particles
Calculating Number of Particles
Number of particles (N) = moles (n) × Avogadro constant (NA)
Rearranged, this also gives: moles = number of particles ÷ NA
Worked Example: Particles from Moles
- Question: how many molecules are there in 0.5 mol of water? [1]
- Answer: N = n × NA = 0.5 × 6.02 × 10²³ = 3.01 × 10²³ molecules [1]
Worked Example: Counting Atoms Within Molecules
- Question: how many hydrogen atoms are there in 1 mol of H₂O? [2]
- Answer: 1 mol of H₂O contains 6.02 × 10²³ molecules [1]; each molecule has 2 H atoms, so total H atoms = 2 × 6.02 × 10²³ = 1.204 × 10²⁴ atoms [1]
The Mole Applies to Any Particle
| Substance | 1 mole contains 6.02 × 10²³ |
|---|---|
| Carbon (C) | atoms |
| Oxygen gas (O₂) | molecules |
| Sodium chloride (NaCl) | formula units |
| Sodium ions (Na⁺) | ions |
| Electrons (e⁻) | electrons |
Common Mistakes
- ❌ Not stating what type of particle a mole quantity refers to
- ✓ always specify atoms, molecules, ions, or formula units - "1 mole of chlorine" is ambiguous unless you say atoms or Cl₂ molecules
- ❌ Forgetting to account for multiple atoms per molecule when counting a specific atom
- ✓ e.g. H₂O has 2 H atoms per molecule - multiply the number of molecules by 2 to get the number of H atoms
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