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Calculating Relative Atomic Mass (Edexcel GCSE Chemistry: Combined Science)
Revision Note
Calculating Relative Atomic Mass
- The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element
- The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present
- So, if there were 3 isotopes present then the top line of the equation would read:
(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)
RAM Calculation Formula
Worked example
The table shows information about the isotopes in a sample of rubidium with 72% 85Rb and 28% 87Rb
Use information from the table to calculate the relative atomic mass of this sample of rubidium. Give your answer to one decimal place:
Answer
Relative Atomic Mass = 85.6
Examiner Tip
Isotopes are easy to recognise from their notation as they have the same symbol but different mass numbers. For example, the two stable isotopes of copper are 63Cu and 65Cu
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