Question

There are three isotopes of oxygen, each containing $8$,$9$,$10$ neutrons respectively. The abundance of one isotope (containing $8$ neutrons) is $90$%. The percentage of other isotope, if the atomic mass of oxygen is $16.12$, are :

• A. ${}^{17}O=8$%,${}^{18}O=2$%
• B. ${}^{17}O=2$%,${}^{18}O=8$%
• C. ${}^{17}O=10$%,${}^{18}O=0$%
• D. ${}^{17}O=4$%,${}^{18}O=6$%

Answer 1

Answer: (A)

${}^{17}O=8$%,${}^{18}O=2$%

According to the given data,
Mass number of isotope of O with 8 neutrons = $16$ and it has $90$% abundance.
Mass number of isotope of O with 9 neutrons = $17$ and let the abundance be $a$%.
Mass number of isotope of O with 10 neutrons = $18$ and its abundance = $(10-a)$%.
Average atomic mass of O
=$\frac{\%of{O}^{16}\times mass+\%of{O}^{17}\times mass+\%of{O}^{18}\times mass}{100}$
$16.12=\frac{90\times 16+17\left(a\right)+18(10-a)}{100}$
$a=8$
% of $O^{17}=8$%
% of $O^{18}=10-8=2$%