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*Revision Materials* 1 Atomic Structure 2 Atoms, molecules and stoichiometry 3 Chemical Bonding 4 States of matter 5 Chemical energetics 6 Electrochemistry 7 Equilibria 8 Reaction kinetics 9 The Periodic Table, chemical periodicity 10 Group 2 11 Group 17 12 Nitrogen and sulfur 13 Organic 14 Hydrocarbons 15 Halogen compounds 16 Hydroxy compounds 17 Carbonyl compounds 18 Carboxylic acids and derivatives 19 Nitrogen compounds 20 Polymerisation 21 Organic synthesis 22 Analytical techniques 23 Chemical energetics 24 Electrochemistry 25 Equilibria 26 Reaction kinetics 27 Group 2 28 Chemistry of transition elements 29 Organic 30 Hydrocarbons 31 Halogen compounds 32 Hydroxy compounds 33 Carboxylic acids and derivatives 34 Nitrogen compounds 35 Polymerisation 36 Organic synthesis 37 Analytical techniques

2 Atoms, molecules and stoichiometry

2.1 Relative masses of atoms and molecules 2.2 The mole and the Avogadro constant 2.3 Formulas 2.4 Reacting masses and volumes (of solutions and gases)

The Mole and the Avogadro Constant

Specification Reference Physical Chemistry: Atoms, molecules and stoichiometry 2.2

Quick Notes

  • Mole: the amount of substance that contains the same number of particles as there are atoms in 12 grams of carbon-12.
  • Avogadro constant (NA) = 6.022 × 1023 particles per mole.
  • The mole allows us to count particles by weighing substances.

Full Notes

Definition of the Mole

A mole is the standard unit for measuring the amount of substance.

Definition:
One mole is the amount of substance that contains 6.022 × 1023 particles. This number is called the Avogadro constant (NA).

These particles could be:

Example 1 mol of H2O contains 6.022 × 1023 water molecules.
1 mol of Na+ contains 6.022 × 1023 sodium ions.

Relationship to Carbon-12

The mole is defined based on the isotope carbon-12:

This gives a link between the mole, mass, and number of particles.

Using the Mole in Calculations

You can use this relationship to convert between number of particles, moles and the Avogadro Constant:

CIE A-Level Chemistry diagram showing relationship between moles, mass, Avogadro constant, and number of particles.

Why the Mole Is Useful

Atoms and molecules are incredibly small, so chemists use the mole to deal with quantities of particles in a practical way — by weighing substances in grams rather than counting individual atoms.

Summary