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1.26 Formula masses

 1.26 "Counting" by measuring mass

Large numbers of coins can be counted by measuring their mass

Sometimes chemists need to "count" out a specific number of atoms, ions or molecules of a substance. Atoms are very very small - too small to see. It is therefore impossible to measure them out by counting individually.  Instead we "count" them out by measuring their mass - much like the way in which coins are counted out in banks by using weighing machines.

We know the Relative atomic masses ( or Ar) of atoms.  This is usually  shown next to the symbol of the element  on the periodic table. 

Assumed background knowledge:


1.14 - 1.17 Atomic structure

If a nucleus was the size of a raisin, the rest of the atom would be the size of a sports stadium

Relative atomic masses are usually shown next to the element's symbol on the  periodic table.

 Students should:

1.26 calculate relative formula masses (including relative molecular masses) (Mr) from relative atomic masses (Ar)

 1.26 Calculating formula mass

The mass of one mole of a compound can be calculated using the relative atomic masses of the component elements multiplied by the number of atoms shown in the formula. 

  Sodium Chloride
    Carbon dioxide

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