Question

Oxygen ( O ) has three naturally occurring isotopes: O16, O17, and O18. O16 is the most abundant isotope with an abundance of 99.757% and an isotopic mass of 15.995 u. The second most abundant isotope is O18 with an abundance of 0.205% and an isotopic mass of 17.999 u. The least abundant isotope, O17, has an abundance of only 0.038% and an isotopic mass of 16.999 u. Calculate the average atomic mass of oxygen.

Answer 1

First, we must check that the sum of the percent abundance of the isotopes is equal to 100.

`\begin{gathered} \%O_(16)+\%O_(17)+\%O_(18)=100\% \\ 99.757\%+0.038\%+0.205\%=100\% \end{gathered}`

Indeed the sum is equal to 100. Now, the average atomic mass will be equal to the sum of the mass of each isotope multiplied by its percentage abundance. Therefore the atomic mass will be equal to:

`\begin{gathered} (m_(O16)*\%O_(16)+m_(O17)*\%O_(17)+m_(O18)*\%O_(18))/(100) \\ (15.995u*99.757\%+16.999u*0.038\%+17.999*0.205\%)/(100) \\ \text{Average atomic mass=15.999u} \end{gathered}`

So, the average atomic mass of oxygen will be 15.999u