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The Basics[]

The Relative Atomic Mass is usually abbreviated to Ar.

As it was at GCSE it is is the mass of an atom found by looking on a suitable Periodic Table.

It is measured in grams per mole (g/mol) but aside from that the only difference is that the Ar's on an A' level Periodic Table will generally be quoted to one decimal place to account for the abundance of isotopes.

RelativeAtomicMass
Bmasspec

The Relative Atom Mass will be between the two Relative Isotopic Masses of 10 and 11.

For an example of how to calculate the Relative Atomic Mass see Isotope.

Exam Hint[]

  • Although you'll have to use Ar's all the time the only question about Ar's is likely to be asking for a definition.
  • In which case make sure you include all the relevent ( underlined) points in the following:
The weighted mean mass of an atom relative to 1/12 the mass of a 12C atom
  • Other than that, make sure you know and can manipulate the formula:
Ar = Mass of element/ Moles of element
  • Although, you would have to take care using the formula above with elements that exist as simple molecules. The wording of such a question is important.

eg.

How many moles of Chlorine gas (Cl2) is found in a 140g sample.

Answer Moles of Cl2 = Mass of Cl2 / MrCl2)
Moles of Cl2 = 140g / 70 g/mol
Moles of Cl2 = 2

However, if it asked how many moles of Chlorine atoms are in 140g the answer would be 4, since each mole of Cl2 contains 2 moles of Cl atoms

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