Question

Chlorine has two isotopes: Cl-35 in 75.5% and Cl-37 in 24.5%. Calculate its atomic mass

Answer 1

We know that the atomic mass of Cl-35 is 34.969 amu and its abundance is 75.5%. The atomic mass of Cl-37 is 36.966 amu and its abundance is 24.5%.

To find the average atomic mass, first, we multiply each percentage by the atomic mass of each isotope.

`\begin{gathered} Cl-35\colon34.969*0.755=26.4amu \\ Cl-37\colon36.966*0.245=9.1\text{amu} \end{gathered}`

Then, add them.

`26.4\text{amu}+9.1\text{amu}=35.5\text{amu}`

Observe that the atomic mass we found is closer to the isotope Cl-35, this is because this isotope has a greater percentage of occurrence in nature.

Therefore, its atomic mass is 35.5 amu.