Question

Question 11
The average atomic mass of a sample of an element $X$ is $16.2u.$ What are the percentages of isotopes ${}_8^{16}X$ and ${}_8^{18}X$ in the sample?

Answer 1

Let the percentage of ${}_8^{16}X$ be A
Then the percentage of ${}_8^{18}X$ $=100-A$

The formula for Calculating Average Atomic Mass

Average Atomic Mass = $\frac{atomicmassofIisotope\times its\%abundance+atomicmassofIIisotope\times its\%abundance}{100}$

The average atomic mass of the given element = 16.2u
Then we have $\frac{16\times A}{100}+\frac{\left(\right(100-A)\times 18)}{100}=16.2$
$Or,1800-1620=18A-16A$
180 = 2A
$A=90$
${}_8^{16}X$ is 90% and is ${}_8^{18}X$ is 10% (100-90 =10)