CALCULATING AVERAGE
ATOMIC MASSUsing Isotope Data to Find a
Weighted Average
ISOTOPES Each isotope will have two values
associated with it.Mass of IsotopePercent Abundance (% found in nature)
Units for Atomic Mass:AMU = atomic mass unit
Weighted average
TO CALCULATE Step #1: For each isotope: (contributing
mass)
Mass of isotope X (% abundance/100)
Step #2: Add products from Step #1
EXAMPLE #1Carbon has two naturally occurring isotopes.
Carbon-12 has a mass of 12.000-amu with an abundance of 98.890%. Carbon-13 has a mass of 13.003-amu with an abundance of 1.11%. Calculate the average atomic mass of carbon.
EXAMPLE #2USING THE DATA ON THE TABLE BELOW, CALCULATE THE AVERAGE ATOMIC MASS OF OXYGEN.
Isotope % Abundance Mass (amu)
Oxygen-16 99.759 15.995
Oxygen-17 0.04 16.115
Oxygen-18 0.21 17.999
REVIEW PROBLEM
Calculate the average atomic mass of copper using the isotope data in the above table.
Isotope % Abundance Mass (amu)
Copper-63 69.20 62.93
Copper-65 30.80 64.93
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