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Relative Formula mass

Relative Formula mass

  • Atomic Mass = A
  • Ar = the relative atomic mass (usually the same as the atomic mass number)
  • When an element has more than one isotope then the relative atomic mass number is the weighted average
    • Weighted mass means that it is measured by how much of the substance there is (so if there is much more of substance C than there is of C1 then the average would be closer to C)
  • The relative formula mass is Mr – you add together the relative atomic masses of all the elements in a substance (e.g. H2O – H=1 O=16 so the relative formula mass is 18)
  • One mole of a substance is Ar or Mr in grams – so O2 (oxygen) Mr = 2×16=32 so one mole = 32g
  • Number of moles = the mass / Mr
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