Question

Oxygen occurs in nature as a mixture of isotopes ${}^{1}6O$, ${}^{1}7O$ and ${}^{1}8O$ having atomic masses of 15.995 u, 16.999 u and 17.999 u and relative abundance of 99.763%, 0.037%, and 0.200% respectively. What is the average atomic mass of oxygen?

• A. 15.999 u
• B. 16.999 u
• C. 17.999 u
• D. 18.999 u

Answer 1

The correct option is A 15.999 u

Average atomic mass of an element existing in different isotopes is given by:

$M_{avg}=\frac{{\sum }_{i=1}^n{M}_i{A}_i}{{\sum }_{i=1}^n{A}_i}$

where $M_i=$atomic mass of an isotope with relative abundance of $A_i$

Given:$M_1=15.995u,A_1=99.763,M_2=16.999u,A_2=0.037,M_3=17.999u,A_3=0.200$

on subtitutiing we get:

$M_{avg}=\frac{15.995\times 99.763+16.999\times 0.037+17.999\times 0.200}{99.763+0.037+0.200}$

$M_{avg}=15.999u$

option A is correct