IGCSEChemistryOxford AQARevision NotesQuantitative ChemistryUse of Amount of Substance In Relation to Masses of Pure SubstancesRelative Formula Mass

Relative Formula Mass (Oxford AQA IGCSE Chemistry): Revision Note

Exam code: 9202

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Updated on

Oxford AQA ChemistryOxford AQA Combined Science Double Award: Chemistry

Relative Formula Mass

  • The symbol for the relative formula mass and relative molecular mass is Mr

    • Relative formula mass is used when referring to the total mass of an ionic compound

  • It is the total mass of the molecule

  • To calculate the Mr of a substance, you have to add up the relative atomic masses of all the atoms present in the formula

Relative Formula Mass Calculations Table

Substance

Atoms present

Calculation

Mr 

Hydrogen
H2 

2 x H

(2 x 1)

2

Water
H2O

(2 x H) + (1 x O)

(2 x 1) + (1 x 16)

18

Potassium carbonate
K2CO3 

(2 x K) + (1 x C) + (3 x O)

(2 x 39) + (1 x 12) + (3 x 16)

138

Calcium hydroxide
Ca(OH)2 

(1 x Ca) + (2 x O) + (2 x H)

(1 x 40) + (2 x 16) + (2 x 1)

74

Ammonium sulfate
(NH4)2SO4

(2 x N) + (8 x H) + (1 x S) + (4 x O)

(2 x 14) + (8 x 1) + (1 x 32) + (4 x 16)

132

Worked Example

Calculate the relative formula mass of:

  1. Sodium chloride, NaCl

  2. Copper oxide, CuO

  3. Magnesium nitrate, Mg(NO3)2

Answers:

  1. Sodium chloride

    • NaCl = 23 + 35.5 = 58.5

  2. Copper oxide

    • CuO = 63.5 + 16 = 79.5

  3. Magnesium nitrate

    • Mg(NO3)2 = 24 + (14 x 1 x 2) + (16 x 3 x 2) = 148

Examiner Tips and Tricks

It is worth showing your working out for these calculations in exams- for more complicated exam questions it can be easy to miscalculate the final value, and you might score one mark for showing your working out.

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Author
Reviewer
Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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