Specification Point 1.26:
Calculate relative formula masses (including relative molecular masses) (Mr) from relative atomic masses (Ar)

Calculating Relative Formula Masses from Relative Atomic Masses

Relative Atomic mass: Given by the symbol Ar

  • Calculated from the mass number and relative abundances of all the isotopes of a particular element
Mass number & atomic number, Edexcel GCSE Chemistry

Example for Isotopes:

The table shows information about the isotopes in a sample of rubidium

Isotope Number of protons Number of neutrons Percentage of isotopes in sample
1 37 48 72
2 37 50 28

Use information from the table to calculate the relative atomic mass of this sample of rubidium. Give your answer to one decimal place

    ( 72   x   85 )   +   ( 28   x   87 )

    ____________________________     =   85.6

                         100

    Relative Atomic Mass = 85.6

Relative formula mass: Given by the symbol Mr, is the total mass of the molecule.

  • To calculate the Mr of a substance, you have to add up the Relative Atomic Masses of all the atoms present in the formula

Example:

Relative formula mass table Edexcel IGCSE Chemistry

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.