Question

Oxygen has three naturally occurring isotopes in the following proportions oxygen 16,99.762%(15.994 91 u); oxygen 17, 0.038000% (16.999 13 u); oxygen 18, 0.20000%(17.999 16 u). What is the average atomic mass of oxygen?

Answer 1

Answer

15.999 u

Explanation:

The atomic mass of O 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 percent of the total).

Set up a table for easy calculation

0.997 62 × 15.994 91 u = 15.956 8   u

0.000 38 × 16.999 13 u =  0.006 46 u

0.002 00 × 17.999 16 u =  0.036 00 u

                          TOTAL = 15.999      u

Answer 2

The average atomic mass is 15.9993

Why?

Take the percentage and times it by the amu and then add all the new numbers together to get the average atomic mass:

(0.99762 x 15.99491) + (0.00038 x 16.99913) + (0.002 x 17.9916) = 15.9993