CALCULATING AVERAGE ATOMIC MASS
Working from Mass Number & % Abundance to get Atomic Mass
Information on % and mass of each Isotope will be given
similar to the table shown to the left.

Convert the % to a decimal (if it is not already a decimal)
75.77 %  .7577
24.23 %  .2423

Multiply the Mass Number by the Corresponding % Abundance
35 x .7577 = 26.5195 AMU
37 x .2423 = 8.9651 AMU

Add the results
26.5195 AMU + 8.9651 AMU = 35.485 AMU (Rounded answer)
Isotope
Multiply
Across
% Abundance Mass (AMU)
Subtotal
(AMU)
Cl-35
.7577
35
26.5195
Cl-37
.2423
37
8.9651
Total -
35.485
Add Down
TRY THE SAME WORK SETTING UP A TABLE AS SHOWN HERE
Working from “Exact Weight” & % Abundance to get Atomic Mass
Information on % and mass of each Isotope will be given
similar to the table shown to the left.

Convert the % to a decimal (if it is not already a decimal)
75.77 %  .7577
24.23 %  .2423


Multiply the Mass Number by the Corresponding % Abundance
34.968852 x .7577 = 26.42596 AMU
36.965903 x .2423 = 8.956838 AMU
Add the results
26.42596 AMU + 8.956838 AMU = 35.3828 AMU (Rounded answer)
Isotope
Multiply
Across
% Abundance Mass (AMU)
Subtotal
(AMU)
Cl-35
.7577
34.968852
26.42596
Cl-37
.2423
36.965903
8.956838
Total -
35.3828
Add Down
TRY THE SAME WORK SETTING UP A TABLE AS SHOWN HERE
Internet Resources:
http://www.chemteam.info/Mole/AverageAtomicWeight.html
https://d3jc3ahdjad7x7.cloudfront.net/fxGaDQWE1TSPqzJxM9go75zFU2vngGfk7IwXnzmsXFBgnc
0d.pdf