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.

Question

Caleb Adams · Accepted Answer

The abundance of #""_8^16"O"# is 99.762 %, and the abundance of #""_8^18"O"# is 0.201 %.

Assume you have 100 000 atoms of O. Then you have 37 atoms of #""_8^17"O"# and 99 963 atoms of the other isotopes.

Let x = the number of atoms of #""_8^16"O"#. Then the number of atoms of #""_8^18"O"# = 99 963 - x

One hundred thousand atoms' total mass is

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

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

2.004 x equals 199 123

x equals 99 762 (199 123/2.004).

So there are 99 762 atoms of #""_8^16"O"# or 99.762 %.

The number of #""_8^18"O"# atoms is 99 963 – 99 762 = 201 atoms or 0.201 %.

I hope this is useful.

Avery Adams · Answer

undefined 18/8 O has a percentage abundance of 0.0007% and 16/8 O of 99.963%.