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1.6.4Physical Chemistry

The Mole and the Avogadro Constant — Edexcel AS Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 28 of 42

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Curriculum Equation Key Moles & Stoichiometry FormulasFast Exam Solution
n = m / M_r | N = n × N_A (6.022 × 10²³) | M = n / V | V_gas = n × 22.4 L (STP)
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Part 1 of 6The Mole

1.6.4 The Mole and the Avogadro Constant
Definition
  • One mole is the amount of substance that contains the same number of particles 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.
Particle types
  • The particles can be atoms, molecules, ions or electrons, so the type must always be stated.
Why use it
  • The mole links tiny particles that cannot be counted with masses that can be measured on a balance.

Part 2 of 6Molar Mass and Relative Masses

Molar Mass and Relative Masses
Relative atomic mass
  • The weighted mean mass of an atom of an element compared with one twelfth of the mass of a carbon-12 atom.
Relative molecular mass
  • The sum of the relative atomic masses of all atoms in a molecule.
Molar mass
  • Numerically equal to the relative mass but with the unit g mol⁻¹.
Example
  • Water has relative molecular mass 18.0, so its molar mass is 18.0 g mol⁻¹.
SubstanceRelative massMolar mass (g mol⁻¹)
Na23.023.0
H₂O18.018.0
CO₂44.044.0
NaCl58.558.5

Part 3 of 6Key Relationships

1
Amount in moles n = mass m ÷ molar mass M.
2
Number of particles N = n × 6.02 × 10²³.
3
Rearranged, the mass of one particle is the molar mass divided by 6.02 × 10²³.
4
For compounds, count particles of each type using the formula, so one mole of H₂O contains two moles of hydrogen atoms.
5
Always keep the unit mol⁻¹ with the Avogadro constant so that dimensions work out.

Part 4 of 6Everyday Scale

Counting units
  • A mole of particles is a very large quantity, similar in role to a dozen being twelve items.
Atoms in a molecule
  • A mole of CO₂ molecules contains one mole of carbon atoms and two moles of oxygen atoms.
Ions in a formula unit
  • One mole of MgCl₂ contains one mole of Mg²⁺ ions and two moles of Cl⁻ ions.
Significant figures
  • Give calculated numbers of particles to three significant figures, matching the data.

Part 5 of 6Worked Example

Question

  • How many hydrogen atoms are in 3.60 g of water?
  • (Ar H = 1.0, O = 16.0;
    • Avogadro constant 6.02 × 10²³ mol⁻¹)

  1. Molar mass of H₂O = 2(1.0) + 16.0 = 18.0 g mol⁻¹.
  2. Moles of H₂O = 3.60 ÷ 18.0 = 0.200 mol.
  3. Each water molecule has two hydrogen atoms, so moles of H atoms = 0.400 mol.
  4. Number of H atoms = 0.400 × 6.02 × 10²³ = 2.41 × 10²³.

Answer 2.41 × 10²³ hydrogen atoms

Part 6 of 6Exam Checklist & Tips

1.6.4 – Key Points for the Exam

  • Mole The amount containing 6.02 × 10²³ specified particles.
  • Avogadro constant 6.02 × 10²³ per mole.
  • Molar mass Mass per mole of a substance in g mol⁻¹.
  • Relative atomic mass Mean atomic mass relative to one twelfth of carbon-12.

Common Mistakes

  • Confusing the number of moles of molecules with the number of moles of atoms.
  • Forgetting to state the unit g mol⁻¹ for molar mass.
  • Leaving out the particle type, such as saying one mole of oxygen without stating atoms or molecules.
  • Using the Avogadro constant with the wrong power of ten.

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