International A LevelChemistryOxford AQARevision NotesPhysical ChemistryAmount of SubstanceRelative Atomic Mass & Relative Molecular Mass

Relative Atomic Mass & Relative Molecular Mass (Oxford AQA International A Level Chemistry)

Revision Note

Richard Boole

Written by: Richard Boole

Reviewed by: Stewart Hird

Relative Masses

Atomic Mass Unit

  • The mass of a single atom is so small that it is impossible to weigh it directly

  • Atomic masses are therefore defined in terms of a standard atom which is called the unified atomic mass unit

  • This unified atomic mass is defined as one-twelfth of the mass of a carbon-12 isotope

  • The symbol for the unified atomic mass is u (often Da, Dalton, is used as well)

  • 1 u = 1.66 x 10-27 kg

Relative atomic mass, Ar

  • The relative atomic mass (Ar) of an element is the ratio of the average mass of the atoms of an element to the unified atomic mass unit

  • The relative atomic mass is determined by using the average mass of the isotopes of a particular element

  • The Ar has no units as it is a ratio and the units cancel each other out

Relative atomic mass of X = fraction numerator average space mass space of space one space atom space of space bold X over denominator one space twelfth space of space the space space mass space of space one space carbon minus 12 space atom end fraction 

  • Relative atomic mass can also be calculated from the abundance of different isotopes

Examiner Tips and Tricks

A more common definition of relative atomic mass is the mass of an atom compared to 1/12th of the mass of a carbon-12 atom

Relative molecular mass, Mr

  • The relative molecular mass (Mr) is the ratio of weighted average mass of a molecule of a molecular compound to the unified atomic mass unit

  • Mr has no units

M subscript straight r equals fraction numerator weighted space average space mass space of space molecules space in space straight a space given space sample space of space straight a space molecular space compound over denominator unified space atomic space mass space unit end fraction

  • Mr can be found by adding up the relative atomic masses of all atoms present in one molecule

  • When calculating Mr the simplest formula for the compound is used, also known as the formula unit

    • Eg. silicon dioxide has a giant covalent structure, however the simplest formula (the formula unit) is SiO2

Example Mr calculations

Substance

Atoms present

Mr

Hydrogen
H2

2 x H

(2 x 1.0) = 2.0

Water
H2O

(2 x H) + (1 x O)

(2 x 1.0) + (1 x 16.0) = 18.0

Silicon dioxide
SiO2

(1 x Si) + (2 x O)

(1 x 28.1) + (2 x 16.0) = 60.1

Potassium carbonate
K2CO3

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

(2 x 39.1) + (1 x 12.0)
+ (3 x 16.0) = 138.2

Calcium hydroxide
Ca(OH)2

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

(1 x 40.1) + (2 x 16.0)
+ (2 x 1.0) = 74.1

Ammonium sulfate
(NH4)2SO4

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

(2 x 14.0) + (8 x 1.0) + (1 x 32.1) + (4 x 16.0) = 132.1

Relative formula mass, Mr 

  • The term relative formula mass (Mr) is used for ionic compounds

  • It has the same units and is calculated in the same way as the relative molecular mass

  • In the table above, the Mr for potassium carbonate, calcium hydroxide and ammonium sulfates are relative formula masses

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    Richard Boole

    Author: Richard Boole

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.

    Stewart Hird

    Author: Stewart Hird

    Expertise: Chemistry Lead

    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.