How to Find the Average Isotopic Mass
- 1). Multiply the first isotope's mass by its percent abundance. The percentage should be expressed in decimal form. For example, 55 percent would be written as 0.55. Write down the result, and mark it as "isotope 1" or something similar so you don't lose track.
- 2). Multiply the second isotope's mass by its percent abundance, and label the result as "isotope 2." Repeat until you have performed this calculation for all of the naturally occurring isotopes of the element in question.
- 3). Add together all of the labeled results from the previous steps. The result is the average isotopic mass.
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