The chlorine present in PVC has two stable isotopes. ³⁵Cl with a mass of 34.97 amu makes up 75.77% of the natural chlorine found. The other isotope is ³⁷Cl, whose mass is 36.95 amu. What is the atomic weight of chlorine?

Strategy To determine the atomic weight, we must calculate the average mass weighted by the fractional abundance of each chlorine isotope. Because there are only two stable isotopes, their abundances must add up to 100%. So we can calculate the abundance of ³⁷Cl from the given abundance of ³⁵Cl.