How do you calculate atomic mass from isotopic composition ?

By calculating the average mass of an element's atoms, you can determine its atomic mass.

Based on the percentage abundances of each isotope, the masses of all the isotopes in an element are weighted to determine its atomic mass.

You take an arbitrary number of atoms, figure out how many of each isotope there are, add up all of their masses, and finally find the average mass.

AN EXAMPLE

If the atomic mass of Cl-35 is 34.969 u and the atomic mass of Cl-37 is 36.965 u, and you are given a sample that contains 75.77% chlorine-35 atoms and 24.23% chlorine-37 atoms, what is the atomic mass of chlorine?

Resolution:

Assuming an arbitrary number of 10,000 Cl atoms, we obtain: 7577 Cl-35 atoms and 2423 Cl-37 atoms.

7577 atoms × (34.969 u)/(1 atom) = 264 960 u is the mass of 7577 Cl-35 atoms.

2423 atoms × (36.965 u)/(1 atom) = 89 566 u is the mass of 2423 Cl-37 atoms.

Cl atom mass of 10,000 = 354 526 u

A Cl atom's average mass is equal to (354 526 u) / (10 000 atoms), or 35.45 u.

Since we were only given four significant figures for the isotope percentages, we are only permitted to use four in our response.

Add the results of multiplying each isotope's mass by its abundance.