Atomic Mass and Atomic Abundance Example Chemistry Problem

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Here is an atomic abundance example chemistry problem:

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%.

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