Topic T5.3 Quantitative Chemistry


4.1 Relative masses and moles

  • we use the carbon atom of carbon-12 as a standard atom and compare the masses of all other atoms with carbon-12
  • The relative atomic mass (Ar) takes into account the proportions of any isotopes of the elementfound naturally. So it is the mean (average) relative mass of the atoms of an element compared with the standard carbon atom (which is assigned a relative value of exactly 12). Therefore the relative atomic mass is not always a whole number, for example, the Ar of chlorine is 35.5.

Calculating relative atomic mass

Chlorine has two isotopes, Chlorine-35 (percentage adundance = 75%) and Chlorine-37 (percentage abundance = 25%). Calculate the relative atomic mass Ar of chlorine.

Ar of Cl = (35 x 75) + (37 x 25) / 100 = 35.5

  • the relative atomic mass (Mr) 


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