3.3 The Mole and the Avogadro Constant

2026 Syllabus Objectives

  1. Core: State that concentration can be measured in g/dm3\text{g/dm}^3 or mol/dm3\text{mol/dm}^3
  2. Supplement: State that the mole, mol, is the unit of amount of substance and that one mole contains 6.02×10236.02 \times 10^{23} particles, e.g. atoms, ions, molecules; this number is the Avogadro constant
  3. Supplement: Use the relationship amount of substance (mol)=mass (g)molar mass (g/mol)\text{amount of substance (mol)} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}} to calculate:
    • (a) amount of substance
    • (b) mass
    • (c) molar mass
    • (d) relative atomic mass or relative molecular/formula mass
    • (e) number of particles, using the value of the Avogadro constant
  4. Supplement: Use the molar gas volume, taken as 24 dm324 \text{ dm}^3 at room temperature and pressure, r.t.p., in calculations involving gases
  5. Supplement: Calculate stoichiometric reacting masses, limiting reactants, volumes of gases at r.t.p., volumes of solutions and concentrations of solutions expressed in g/dm3\text{g/dm}^3 and mol/dm3\text{mol/dm}^3, including conversion between cm3\text{cm}^3 and dm3\text{dm}^3
  6. Supplement: Use experimental data from a titration to calculate the moles of solute, or the concentration or volume of a solution
  7. Supplement: Calculate empirical formulae and molecular formulae, given appropriate data
  8. Supplement: Calculate percentage yield, percentage composition by mass and percentage purity, given appropriate data

Chemical Accountancy and Stoichiometry

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Stoichiometry is derived from two Greek words – stoicheion (meaning element) and metron (meaning measure). It describes the relative measures or amounts of a reactant and a product in a chemical reaction.

🔑 Key Concept: Stoichiometry is the link between what happens at an atomic level and what can be measured practically in a reaction.

Although the pairing of elements may change in a chemical reaction, the amount of matter remains the same. Therefore, stoichiometry is a type of chemical accountancy that takes place at the atomic level.

The Mole as a Standard Unit

The standard amount of substance that contains a known number of particles at an atomic level is known as the mole. The concept of the mole is important to:

  • Understanding chemical reactions quantitatively
  • Industrial chemistry and Green Chemistry principles
  • Calculating yields and atom economy
  • Measuring amounts of substances in different situations

Green Chemistry Principle: The atom economy of a reaction measures the percentage of substance present in the reactants that finishes up in the desired product.

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