An average sample of chlorine contains 75.77% chlorine-35, with an atomic mass of 34.969 amu (atomic mass units), and 24.23% chlorine-37, with an atomic mass of 36.966 amu. What is the average atomic mass of chlorine?
You require the weighted average.
The weighted average is the sum of each individual nuclide, multiplied by its isotopic abundance.
So, we simply do the arithmetic:
Then look at the Periodic Table to see how accurate your result is.