Answer :
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.
[tex]\begin{gathered} Cl-35\colon34.969\times0.755=26.4amu \\ Cl-37\colon36.966\times0.245=9.1\text{amu} \end{gathered}[/tex]Then, add them.
[tex]26.4\text{amu}+9.1\text{amu}=35.5\text{amu}[/tex]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.