Question #5ae01

1 Answer
Mar 18, 2015

You need two things in order to calculate the average atomic mass of chlorine (or any element for that matter): the exact weight of all its naturally-occuring isotopes, and the abundance of each of those isotopes.

In chlorine's case, you have two naturally-occuring isotopes, each with a different mass number - chlorine-35 and chlorine-37.

The exact weight and the abundance of these isotopes are

#""^35"Cl": "34.968852 amu"# #-># #"75.77% abundance"#
#""^37"Cl": "36.965903 amu"# #-># #"24.23% abundance"#

The procedure is very simple - you just multiply the weight of each isotope by its abundance and add the resulting values. I'll use decimal abundance, which expresses the abundance as numbers between 0 and 1, for simplicity.

To get decimal abundance, just divide the percent abundance by 100.

So, the average atomic mass of chlorine will be

#"(weight of Cl-35" * "abundance Cl-35") + ("weight of Cl-37" * "abundance Cl-37")#

#34.968852 * 0.7577 + 36.965903 * 0.2423 = 35.4527#

The average atomic mass of chlorine is 35.4527 amu.