How to Calculate Atomic Abundance from Atomic Mass

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?

Solution

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

Answer

The atomic abundance of ¹¹₅B is 80%.