# Question #5ae01

##### 1 Answer

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

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

The average atomic mass of chlorine is **35.4527 amu**.