1.1.3 Relative Mass

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Relative Mass
Relative masses are masses of atoms compared to carbon-12. The actual mass of an atom is
very, very tiny. So, the mass of one atom is compared to the mass of a different atom. This is its
relative mass.
The Relative Atomic Mass, Ar, is the average mass of an atom of an element compared to 1/
12 of the mass of an atom of carbon-12
The Relative Isotopic Mass is the mass of an atom if an isotope of an element compared to 1/
12 of the mass of an atom of carbon-12
The Relative Molecular Mass, Mr, is the average mass of a molecule or formula unit compared
to 1/12 of the mass of an atom of carbon-12
Ar can be worked out from isotopic abundances. Different isotopes of an element occur in
different isotopic abundances. For example, 76% of chlorine atoms found on earth have
a relative isotopic mass of 35, while 24% have a relative isotopic mass of 37. The relative
isotopic mass of chlorine is the average mass of all chlorine atoms. If you've got the isotopic
abundances as percentages, the easiest way to calculate relative atomic mass is to:
Step 1: Multiply each relative isotopic mass by its % relative isotopic abundance and add up the
results- (76x35) + (24x37) = 2660 + 888 = 3548
Step 2: Divide by 100- 3548/100 = 35.5
So the relative atomic mass of chlorine is 35.5.

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