AQA GCSE Chemistry C2 3.2 Masses of atoms and moles

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Relative atomic mass & Formula mass

  • The actual mass of an atom is so small it wouldn't be useful to use in experiments/calculations, so instead the relative mass is used; (Ar)
  • E.g.: We use the atom of carbon-12 as a standard for carbon-it gives a mass of exactly 6 protons and 6 neutrons, so it has a 'mass' of 12 units. This is used to compare to other relative atomic masses of other atoms.
  • The relative atomic mass of an element is usually the same/similar to the mass number of its most common isotopes of the element which is found naturally-an average mass.
  • Relative formula masses (Mr) of compounds:
  • E.g.: Sodium Chloride; NaCl.
    • Ar of Na=23
    • Ar of Cl=35.5
    • Mr=23+35.5=58.5
  • E.g.: CO2
    • Ar of C=12
    • Ar of O=6 there are two oxygen atoms in the compound so 6x2
    • Mr=12+6x2=24
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  • This is sort of a shorthand of 'relative atomic mass in grams'.
  • For example, in an Oxgen atom, with a relative atomic mass of 6, the relative atomic mass in grams has exactly one mole of oxgen atoms.
  • A mole is equal to 6.02 x 10^23.
  • This means that in 6g of oxygen atoms, there are 6.02 x 10^23 oxygen atoms.
  • The same way, the relative atomic mass for carbon in grams is 12g, which means there is a mole of carbon atoms in 12g of it.
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