Question

Oxygen exists in three isotopic forms with mass numbers 16, 17, and 18. The percentage abundances of these isotopes are $81\%$, $9\%$, and $10\%$, respectively. Calculate the average atomic mass of the element.

• A. 16.9
• B. 17.2
• C. 16.29
• D. 16.5

Answer 1

The correct option is C

16.29

Explanation of the correct answer

Average Atomic Mass:

1. The total of the masses of an element's isotopes, each multiplied by its natural abundance, is its average atomic mass (the decimal associated with the percent of atoms of that element that are of a given isotope).
2. Average atomic mass = $f_1{M}_1+f_2{M}_2+...+f_n{M}_n$, where f is the fraction representing the isotope's natural abundance and M is the isotope's mass number (weight).

Given Percentage of isotopes:

$$\begin{gathered} \%of{O}^{16}=81\% \\ \%of{O}^{17}=9\% \\ \%of{O}^{18}=10\% \end{gathered}$$

The average atomic mass of oxygen = $O^{16}+O^{17}+O^{18}$

$$\begin{gathered} =16\times \frac{81}{100}+17\times \frac{9}{100}+18\times \frac{10}{100} \\ =16\times 0.81+17\times 0.09+18\times 0.1 \\ =16.29 \end{gathered}$$

Hence, the correct option is (c).