# Oxygen is composed of three isotopes 16/8 O (15.995 u), 17/8 O (16.999 u) and 18/8 O (17.999 u). One of these isotopes, 17/8 O, comprises of 0.037% of oxygen. What is the percentage abundance of the other two isotopes, using the average atomic mass of 15.9994 u.

Jun 23, 2014

The abundance of $\text{_8^16"O}$ is 99.762 %, and the abundance of $\text{_8^18"O}$ is 0.201 %.

Assume you have 100 000 atoms of O. Then you have 37 atoms of $\text{_8^17"O}$ and 99 963 atoms of the other isotopes.

Let x = the number of atoms of $\text{_8^16"O}$. Then the number of atoms of $\text{_8^18"O}$ = 99 963 - x

The total mass of 100 000 atoms is

x × 15.995 u + (99 963 – x) × 17.999 u + 37 × 16.999 u = 100 000 × 15.9994 u

15.995 x + 1 799 234.037 – 17.999 x + 628.963 = 1 599 940

2.004 x = 199 123

x = 199 123/2.004 = 99 762

So there are 99 762 atoms of $\text{_8^16"O}$ or 99.762 %.

The number of $\text{_8^18"O}$ atoms is 99 963 – 99 762 = 201 atoms or 0.201 %.

Hope this helps.