Answer: The average atomic mass of oxygen is 15.999 amu
Explanation:
Mass of isotope O-16 = 15.995 amu
% abundance of isotope O-16= 99.759 % = 
Mass of isotope O-17 = 16.995 amu
% abundance of isotope O-17 = 0.037% = 
Mass of isotope O-18 = 17.999 amu
% abundance of isotope O-18 = 0.204% = 
Formula used for average atomic mass of an element :

![A=\sum[(15.995\times 0.99759)+(16.995\times 0.00037)+(17.999 \times 0.00204)]](https://tex.z-dn.net/?f=A%3D%5Csum%5B%2815.995%5Ctimes%200.99759%29%2B%2816.995%5Ctimes%200.00037%29%2B%2817.999%20%5Ctimes%200.00204%29%5D)

Thus the average atomic mass of oxygen is 15.999 amu
To go from molecules to moles divide by Avogadro's number [<span>6.02x10^23]</span>
Example:
1.50x10^23 divided by 6.02x10^23 = 0.249 moles rounded to three significant figures
Generally, the top elements in group 16 will have a charge of -2, although the entire group isn't standardized.