1.26 "Counting" by measuring 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:
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.