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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; the extra named ions, hydrated salts and limiting reagents follow with the supplementary set.

Exam Papers
Papers 1 & 2
9701/1 and 9701/2 (AS Level)
Learning Outcomes
2.1.1 – 2.4.1
4 parts
Topic Parts
4 of 4 live
2.1, 2.2, 2.3, 2.4
Exam Board
Cambridge
CIE 9701 (2025-2027)

Revision Notes

Work through each part of Cambridge Topic 2 in a structured sequence.

Foundations
Atoms, Elements
and Molecules
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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Formulae
Ions and
Ionic Formulae
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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Water of crystallisation
Hydrated
Salts
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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Equations
Equations and
Reaction Types
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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Formulae
Empirical and
Molecular
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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Calculations
Mole
Calculations
6

2.4 Mole Calculations

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

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Gases and solutions
Solutions
and Gases
7

2.4 Solutions and Gases

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

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Specification Coverage

Topic 2 Atoms, Molecules and Stoichiometry – Cambridge International AS Level Chemistry

The following Cambridge learning outcomes state what candidates should be able to do for Topic 2. Wording is taken from the Cambridge International AS & A Level Chemistry 9701 syllabus (2025-2027).

2.1 Relative masses of atoms and molecules

2.1.1
define the unified atomic mass unit as one twelfth of the mass of a carbon-12 atom
2.1.2
define relative atomic mass, Ar, relative isotopic mass, relative molecular mass, Mr, and relative formula mass in terms of the unified atomic mass unit

2.2 The mole and the Avogadro constant

2.2.1
define and use the term mole in terms of the Avogadro constant

2.3 Formulas

2.3.1
write formulas of ionic compounds from ionic charges and oxidation numbers (shown by a Roman numeral), including:
(a)
the prediction of ionic charge from the position of an element in the Periodic Table
(b)
recall of the names and formulas for the following ions: NO3–, CO32–, SO42–, OH–, NH4+, Zn2+, Ag+, HCO3–, PO43–
2.3.2
(a)
write and construct equations (which should be balanced), including ionic equations (which should not include spectator ions)
(b)
use appropriate state symbols in equations
2.3.3
define and use the terms empirical and molecular formula
2.3.4
understand and use the terms anhydrous, hydrated and water of crystallisation
2.3.5
calculate empirical and molecular formulas, using given data

2.4 Reacting masses and volumes (of solutions and gases)

2.4.1
perform calculations including use of the mole concept, involving:
(a)
reacting masses (from formulas and equations) including percentage yield calculations
(b)
volumes of gases (e.g. in the burning of hydrocarbons)
(c)
volumes and concentrations of solutions
(d)
limiting reagent and excess reagent
(e)
deduce stoichiometric relationships from calculations such as those in 2.4.1(a)–(d)