The weight of a large # of different isotopes that makes up a sample is termed the Atomic weight of the sample. It is calculated as a weighted average. The procedure for this calculation is as follows.
  1. Express the percentage of each isotope as a decimal number (divide by 100).
  2. Multiply each percentage as a decimal by the corresponding atomic mass (in amu's) of the isotope (isotope fraction).
  3. Add up the isotope fractions.
Example:
What is the atomic weight of a sample of Lithium that consists of the isotopes listed in the table below?
    Isotope
    Atomic Mass(amu)
    Abundance (%)
    Lithium - 6
    6.015
    7.68
    Lithium - 7
    7.016
    92.32
Calculations:
  Atomic Mass   Decimal Percent    
Lithium - 6
6.015
X
.0768
=
0.461952
Lithium - 7
7.016
X
.9232
=
6.477171
Atomic Weight
6.94
(answer rounded to 3 significant figures)
Assignment #3: Calculation of Atomic Weight:
Calculate the Atomic weight of the following samples using the information given.
1) A Sample of Potassium
Isotope
Atomic Mass (amu)
Abundance (%)
potassium - 39
38.964
93.12
potassium - 41
40.962
6.88
2) A Sample of Boron
Isotope
Atomic Mass (amu)
Abundance (%)
Rubidium -85
85.04
72.15
Rubidium -87
86.94
27.85
3) A Sample of Magnesium
Isotope
Atomic Mass (amu)
Abundance (%)
Magnesium - 24
23.986
78.60
Magnesium - 25
24.985
10.11
Magnesium - 26
25.983
11.29
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