Relative Masses
Chemistry: AQA A Level, topic 1.2.1
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.
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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.
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.
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.
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!
Test yourself on Relative Masses
What is an amu?