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OCR A Level Chemistry A – Module 2 Foundations in Chemistry

Empirical and Molecular Formulae

Empirical and molecular formulae connect experimental composition data with the actual formula of a compound. These pages build from finding the simplest whole-number ratio of atoms through to using relative molecular mass to scale up to the true molecular formula.

These pages cover the mole, formulae and every calculation type; hydrated salts and measuring techniques follow with the supplementary set.

Exam Paper
Paper 1 & 2
H432/01 and H432/02
Specification Points
2.1.3(b) – 2.1.3(c)
2 points covered
Sub-topics
2 lessons
Revision notes available
Exam Board
OCR
H432 (2015 onwards)

Revision Notes

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

A single repeating unit lifted from a pattern
1

Empirical Formulae

Revise how to convert masses, percentages or combustion data into mole ratios and then into the simplest whole-number formula.

Start revising
A whole molecule beside its simplest repeating unit
2

Molecular Formulae

Learn how the empirical formula mass and relative molecular mass are used to find the molecular formula of a compound.

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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

Formulae and Hydrated Salts Map

OCR A Level Chemistry A · 2.1.3 Amount of substance · Formulae and hydrated salts

Formulae and hydrated salts on one A4 page: empirical formulae, awkward ratios, 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

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2.1.3

  1. b (i)

    use of the term empirical formula (the simplest whole number ratio of atoms of each element present in a compound)

    Revise
  2. b (ii)

    use of the term molecular formula (the number and type of atoms of each element in a molecule)

    Revise
  3. c i

    calculations of empirical formulae from composition by mass

    Revise
  4. c ii

    calculations of empirical formulae from percentage compositions by mass

    Revise
  5. c iii

    calculations of molecular formulae from composition by mass or percentage compositions by mass and relative molecular mass

    Revise