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Cambridge International AS Level Chemistry

Topic 2
Atoms, Molecules and Stoichiometry

Topic 2 covers relative masses and the mole, the formulae of compounds and balanced equations, empirical and molecular formulae, and calculations using the mole concept with masses, gas volumes and solutions.

These pages cover all four parts, including the extra named ions, hydrated salts and limiting reagents.

Exam Papers
Papers 1 & 2
9701/1 and 9701/2 (AS Level)
Learning Outcomes
2.1.1 – 2.4.1
4 parts
Sub-topics
7 lessons
2.1, 2.2, 2.3, 2.4
Exam Board
Cambridge
CIE 9701 (2025-2027)

Revision Notes

7 revision notes, in the order the specification teaches them.

Single atoms beside a molecule of an element and a compound
1

2.1 / 2.2 Atoms, Elements and Molecules

Relative atomic, molecular and formula mass, molar mass, the mole and the Avogadro constant (2.1, 2.2).

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Ions fitting together in the right ratio
2

2.3 Ions and Ionic Formulae

Ion charges from the Periodic Table, the nine named ions, Roman-numeral oxidation numbers and constructing ionic formulae (2.3.1).

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Blue hydrated crystals beside white anhydrous powder
3

2.3 Hydrated Salts and Water of Crystallisation

Anhydrous and hydrated salts, water of crystallisation and finding the degree of hydration (2.3.4).

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A level balance with the same atoms in each pan
4

2.3 Equations and Reaction Types

Balanced full and ionic equations with state symbols, and the typical reactions of acids (2.3.2).

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A whole molecule beside its simplest repeating unit
5

2.3 Empirical and Molecular Formulae

Empirical formulae from data and molecular formulae from Mᵣ (2.3.3, 2.3.5).

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A heap of powder opening out into countless particles
6

2.4 Mole Calculations

Reacting masses, percentage yield, atom economy and stoichiometric relationships (2.4.1 (a), (e)).

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Two beakers, one dilute and one concentrated
7

2.4 Solutions and Gases

Volumes of gases and volumes and concentrations of solutions (2.4.1 (b), (c)).

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Summary Map

Download, print and revise the key facts of this topic at a glance.

Summary map

Download, print and revise the key facts of this topic at a glance.

Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Atoms, Molecules and Ions Map

Cambridge International AS & A Level Chemistry · Topic 2 Atoms, molecules and ions

The whole section on one A4 page: atoms, elements, molecules and giant structures, ions, the ions to recall and formulae from charges and oxidation numbers.

Download the map6 key ideas + an ions table · A4 portrait · print ready
Illustration of a summary map: compounds joined by reaction arrows
Summary map · PDF

Formulae and Hydrated Salts Map

Cambridge International AS & A Level Chemistry · Topic 2 Empirical formulae and hydrated salts

The whole section on one A4 page: empirical formulae from masses and percentages, molecular formulae, combustion analysis and hydrated salts.

Download the map6 key ideas + a data table · A4 portrait · print ready

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Study Cards

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Specification

Every specification point in this topic, in the exam board's own words. Rate how confident you are with each one.

Official Cambridge documentsSyllabus (opens the Cambridge website in a new tab)

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2.1.1

  1. 2.1.1

    define the unified atomic mass unit as one twelfth of the mass of a carbon-12 atom

    Revise

2.1.2

  1. a

    define relative atomic mass, Ar, in terms of the unified atomic mass unit

    Revise
  2. b

    define relative isotopic mass in terms of the unified atomic mass unit

    Revise
  3. c

    define relative molecular mass, Mr, in terms of the unified atomic mass unit

    Revise
  4. d

    define relative formula mass in terms of the unified atomic mass unit

    Revise

2.2.1

  1. 2.2.1

    define and use the term mole in terms of the Avogadro constant

    Revise

2.3.1

  1. 2.3.1

    write formulas of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral)

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  2. a

    the prediction of ionic charge from the position of an element in the Periodic Table

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  3. b i

    recall of the name and formula for the nitrate ion, NO3–

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  4. b ii

    recall of the name and formula for the carbonate ion, CO32–

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  5. b iii

    recall of the name and formula for the sulfate ion, SO42–

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  6. b iv

    recall of the name and formula for the hydroxide ion, OH–

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  7. b v

    recall of the name and formula for the ammonium ion, NH4+

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  8. b vi

    recall of the name and formula for the zinc ion, Zn2+

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  9. b vii

    recall of the name and formula for the silver ion, Ag+

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  10. b viii

    recall of the name and formula for the hydrogencarbonate ion, HCO3–

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  11. b ix

    recall of the name and formula for the phosphate ion, PO43–

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2.3.2

  1. a i

    write and construct equations (which should be balanced)

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  2. a ii

    write and construct ionic equations (which should not include spectator ions)

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  3. b

    use appropriate state symbols in equations

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2.3.3

  1. a

    define and use the term empirical formula

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  2. b

    define and use the term molecular formula

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2.3.4

  1. a

    understand and use the term anhydrous

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  2. b

    understand and use the term hydrated

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  3. c

    understand and use the term water of crystallisation

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2.3.5

  1. a

    calculate empirical formulas, using given data

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  2. b

    calculate molecular formulas, using given data

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2.4.1

  1. a i

    perform calculations including use of the mole concept, involving reacting masses (from formulas and equations)

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  2. a ii

    perform calculations including use of the mole concept, involving percentage yield calculations

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  3. b

    perform calculations including use of the mole concept, involving volumes of gases (e.g. in the burning of hydrocarbons)

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  4. c

    perform calculations including use of the mole concept, involving volumes and concentrations of solutions

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  5. d i

    perform calculations including use of the mole concept, involving limiting reagent

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  6. d ii

    perform calculations including use of the mole concept, involving excess reagent

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  7. e

    deduce stoichiometric relationships from calculations such as those in 2.4.1(a)–(d)

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