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?

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

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