How do I calculate relative atomic mass?
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Average Atomic Mass Calculation
Example 1.1 BROMINE
o bromine consists of 50% 79Br and 50% 81Br, calculate the Ar of bromine.
o Ar = [ (50 x 79) + (50 x 81) ] /100 = 80 amu
o So the average atomic mass of bromine is 80 amu.
o Note the full working shown. Yes, ok, you can do it in your head BUT many students ignore the %'s
and just average all the isotopic masses (mass numbers) given, in this case bromine-79 and bromine-81.
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Average Atomic Mass Calculation
Example 1.2 CHLORINE
o chlorine consists of 75% chlorine-35 and 25% chlorine-37.
o Think of the data based on 100 atoms, so 75 have a mass of 35 and 25 atoms have a mass of 37 amu.
o The average mass = [ (75 x 35) + (25 x 37) ] / 100 = 35.5
o So the average atomic mass = 35.5 amu
o Note: 35Cl and 37Cl are the most common isotopes of chlorine, but, there are tiny percentages of other
chlorine isotopes which are usually ignored in high school chemistry.
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Please note: The mass number for any isotope is the sum of the protons and neutrons in the nucleus, and is always
an integer ( a whole number.)
1. Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153. Europium-151 has an
abundance of 48.03% and Europium-153 has an abundance of 51.97%. What is the atomic mass of europium?
2.
Calculate the atomic mass of copper if copper-63 is 69.17% abundant and copper-65 is 30.83% abundant.
3. Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87
(abundance of 7.0%), and 88 (abundance of 82.6%). Calculate the atomic mass of strontium.
46
47
48
4. Titanium has five common isotopes: Ti (8.0%), Ti (7.8%), Ti (73.4%),
49Ti (5.5%), 50Ti (5.3%). What is the average atomic mass of titanium?