Question

Three isotopes of oxygen, each containing 8, 9, 10 neutrons respectively has the % of one isotope (containing 8 neutrons) 90%. The abundance of other isotopes is :
(The atomic weight of oxygen is 16.12)

• A. 7%, 3%
• B. 5%, 5%
• C. 8%, 2%
• D. none of these

Answer 1

Answer: (C)

8%, 2%

Mass number of isotope of O with 8 neutrons = 16 and is 90%.
Mass number of isotope of O with 10 neutrons = 18 [(10 - a)%]
Average atomic weight = $16.12$
$16.12=\frac{(90\times 16)+(17\times a)+\left[18\right(10-a\left)\right]}{100}$
$a=8$
% of $O^{17}$ = 8%
% of $O^{18}$ = 10 - 8 =$2$ %