Naturally occurring chlorine that is put in pools is 75.53 percent 35Cl and 24.47 percent Cl. Calculate the average atomic mass.

A_r = 35.5

The average atomic mass of Cl is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i. e., its % abundance).

Thus,

0.7553 * 35 u = 26.4 u

0.2447 * 37 u = 9.05 u

TOTAL = 35.5 u