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
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|
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
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
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