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