How to Calculate Atomic Abundance from Atomic Mass

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In chemistry, one often needs to calculate different forms of measurement. In this example, we calculate atomic abundance from atomic mass

The element boron consists of two isotopes, 105B and 115B. Their masses, based on the carbon scale, are 10.01 and 11.01, respectively. The abundance of 105B is 20.0%.
What is the atomic abundance of and the abundance of 115B?


The percentages of multiple isotopes must add up to 100%.
Since boron only has two isotopes, the abundance of one must be 100.0 - the abundance of the other.

abundance of 115B = 100.0 - abundance of 105B

abundance of 115B = 100.0 - 20.0
abundance of 115B = 80.0


The atomic abundance of 115B is 80%.