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1.3.1Physical Chemistry

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

5 min read 6 parts Lesson 14 of 102

📚 AQA AS Chemistry Hub / Topical Revision
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Curriculum Equation Key قوانين المول والحسابات الكيميائية - 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)
Word Equation & Conditions: عدد المولات = الكتلة بالجرام ÷ الكتلة المولية | عدد الجسيمات = عدد المولات × عدد أفوجادرو
Location & Specifications: كيمياء التحصيلي وثانوي وبكالوريا و Cambridge IGCSE Chemistry.
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Part 1 of 6The Mole

The mole is the unit of amount of substance.

Definition
  • Definition A mole is the amount of substance that contains the same number of particles as there are atoms in exactly 12 g of carbon-12.
Avogadro constant
  • Avogadro constant The Avogadro constant, L, is 6.02 × 10²³ mol⁻¹ and gives the number of particles in one mole.
Types of particle
  • Types of particle The particles can be atoms, molecules, ions or electrons, so the particle type must always be stated.
Molar mass
  • Molar mass One mole has a mass in grams equal to the relative atomic or formula mass, in units of g mol⁻¹.

Part 2 of 6Key Equations

Key Equations
Amount
  • Amount n = m ÷ M, where m is the mass in grams and M is the molar mass in g mol⁻¹.
Particles
  • Particles Number of particles = n × L.
Rearranged
  • Rearranged m = n × M and n = number of particles ÷ L are the other forms of the same equations.
Units
  • Units Always check that mass is in grams and amount in moles before substituting.

Part 3 of 6Counting Different Particles

1
Write the formula and decide which particle you are asked to count.
2
Calculate the moles of the substance using n = m ÷ M.
3
Multiply the moles by L to find the number of formula units or molecules.
4
To count atoms, multiply by the number of atoms of that type in one formula unit.
5
Check that the answer is of a sensible size, with a power of 10 consistent with the amount.

Part 4 of 6Examples of One Mole

Sodium
  • Sodium 1 mol of Na has a mass of 23.0 g and contains 6.02 × 10²³ atoms.
Hydrogen
  • Hydrogen 1 mol of H₂ has a mass of 2.0 g and contains 6.02 × 10²³ molecules, which is 1.204 × 10²⁴ atoms.
Sodium chloride
  • Sodium chloride 1 mol of NaCl has a mass of 58.5 g and contains 6.02 × 10²³ formula units.
Ions
  • Ions 1 mol of CaF₂ contains 1 mol of Ca²⁺ ions and 2 mol of F⁻ ions.

Part 5 of 6Worked Example

Question

  • Calculate the number of molecules and the total number of atoms in 3.60 g of water, H₂O (Ar: H = 1.0, O = 16.0;
    • L = 6.02 × 10²³ mol⁻¹).

  1. M(H₂O) = 2 × 1.0 + 16.0 = 18.0 g mol⁻¹.
  2. n = 3.60 ÷ 18.0 = 0.200 mol.
  3. Molecules = 0.200 × 6.02 × 10²³ = 1.20 × 10²³.
  4. Each molecule has 3 atoms, so atoms = 3 × 1.204 × 10²³ = 3.61 × 10²³.

Answer

  • 1.20 × 10²³ molecules;
    • 3.61 × 10²³ atoms

Part 6 of 6Exam Checklist & Tips

1.3.1 – Key Points for the Exam

  • Mole The amount of substance containing as many particles as there are atoms in 12 g of carbon-12.
  • Avogadro constant The number of particles in one mole, 6.02 × 10²³ mol⁻¹.
  • Molar mass The mass of one mole of a substance, in g mol⁻¹.
  • Formula unit The simplest ratio of ions in an ionic compound.

Common Mistakes

  • Not specifying the type of particle, for example “1 mole of oxygen” instead of oxygen atoms or molecules.
  • Counting only molecules and forgetting to multiply by the number of atoms in each molecule.
  • Leaving out the unit mol⁻¹ from the Avogadro constant.
  • Using the mass in kilograms in n = m ÷ M.

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