1.1.11

Relative Masses

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Relative Atomic Mass

Relative atomic mass (Ar) is the average mass of an element compared to one-twelfth the mass of a single carbon-12 atom.

Illustrative background for What does this mean?Illustrative background for What does this mean? ?? "content

What does this mean?

  • 1amu is defined as one-twelfth of the mass of a single carbon-12 atom.
  • So Ar is the average mass of an element in units of amu.
Illustrative background for What is the average mass?Illustrative background for What is the average mass? ?? "content

What is the average mass?

  • Almost every element has multiple stable isotopes.
    • This means that some atoms of an element will weigh more than others.
  • The average mass is the average of the masses of the elements, weighted according to their abundance.
    • E.g. 35Cl is 75% abundant and 37Cl is 25% abundant.
      • Ar = (0.75 × 35) + (0.25 × 37) = 35.5

Relative Molecular Mass

Relative molecular mass (Mr) is the average mass of a molecule compared to one-twelfth the mass of a single carbon-12 atom.

Illustrative background for What does this mean?Illustrative background for What does this mean? ?? "content

What does this mean?

  • 1amu is defined as one-twelfth of the mass of a single carbon-12 atom.
  • So Mr is the average mass of a molecule in the units of amu.
Illustrative background for What is the average mass?Illustrative background for What is the average mass? ?? "content

What is the average mass?

  • Recall that the Ar takes into account the differences in mass and abundance of isotopes.
  • This means that we can simply add the Ar values for all the atoms in a molecule to calculate the Mr!

Jump to other topics

1Principles of Science I

1.1Structure & Bonding

1.2Properties of Substances

1.3Cell Structure & Function

1.4Cell Specialisation

1.5Tissue Structure & Function

1.6Working with Waves

1.7Waves in Communication

2Practical Scientific Procedures and Techniques

3Science Investigation Skills

4Principles of Science II

4.1Extracting Elements

4.2Relating Properties to use of Substances

4.3Organic Chemistry

4.4Energy Changes in Industry

4.5The Circulatory System

4.6Ventilation & Gas Exchange

4.7Urinary System

4.8Cell Transport

4.9Thermal Physics

4.10Materials

4.11Fluids

5Contemporary Issues in Science

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