Revision NotesExam QuestionsPast Papers
A LevelChemistryAQARevision NotesPhysical ChemistryFormulae, Equations & CalculationsRelative Atomic Mass & Relative Molecular Mass

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

Revision Note

Francesca

Author

Francesca

Last updated

Relative Atomic Mass Definition

What is Relative Atomic Mass?

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 Definition

  • 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 bold a bold v bold e bold r bold a bold g bold e bold space bold m bold a bold s bold s bold space bold o bold f bold space bold o bold n bold e bold space bold a bold t bold o bold m bold space bold o bold f bold space bold X over denominator bold o bold n bold e bold space bold t bold w bold e bold l bold f bold t bold h bold space bold o bold f bold space bold t bold h bold e bold space bold space bold m bold a bold s bold s bold space bold o bold f bold space bold o bold n bold e bold space bold c bold a bold r bold b bold o bold n bold minus bold 12 bold space bold a bold t bold o bold m end fraction

Relative isotopic mass

  • The relative isotopic mass is the mass of a particular atom of an isotope compared to the value of the unified atomic mass unit

  • Atoms of the same element with a different number of neutrons are called isotopes

  • Isotopes are represented by writing the mass number as 20Ne, or neon-20 or Ne-20

    • To calculate the average atomic mass of an element the percentage abundance is taken into account

    • Multiply the atomic mass by the percentage abundance for each isotope and add them all together

    • Divide by 100 to get average relative atomic mass

    • This is known as the weighted average of the masses of the isotopes

Atomic Structure 1, downloadable AS & A Level Chemistry revision notes

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

  • The Mr has no units

Mr = fraction numerator bold w bold e bold i bold g bold h bold t bold e bold d bold space bold a bold v bold e bold r bold a bold g bold e bold space bold m bold a bold s bold s bold space bold o bold f bold space bold m bold o bold l bold e bold c bold u bold l bold e bold s bold space bold i bold n bold space bold a bold space bold g bold i bold v bold e bold n bold space bold s bold a bold m bold p bold l bold e bold space bold o bold f bold space bold a bold space bold m bold o bold l bold e bold c bold u bold l bold a bold r bold space bold c bold o bold m bold p bold o bold u bold n bold d over denominator bold u bold n bold i bold f bold i bold e bold d bold space bold a bold t bold o bold m bold i bold c bold space bold m bold a bold s bold s bold space bold u bold n bold i bold t end fraction

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

  • When calculating the 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

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 relative formula mass (Mr) is used for compounds containing ions

  • 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

You've read 0 of your 5 free revision notes this week

Sign up now. It’s free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:Ionisation Energy: Trends & EvidenceNext:Empirical & Molecular Formula
    Francesca

    Author: Francesca

    Expertise: Head of Science

    Fran studied for a BSc in Chemistry with Forensic Science, and since graduating taught A level Chemistry in the UK for over 11 years. She studied for an MBA in Senior Leadership, and has held a number of roles during her time in Education, including Head of Chemistry, Head of Science and most recently as an Assistant Headteacher. In this role, she used her passion for education to drive improvement and success for staff and students across a number of subjects in addition to Science, supporting them to achieve their full potential. Fran has co-written Science textbooks, delivered CPD for teachers, and worked as an examiner for a number of UK exam boards.