Question

The relative atomic mass of an element $A$ is $16.2$. There are two isotopes ${}_8^{16}A$ and ${}_8^{18}A$ of the element. Calculate the percentage of ${}_8^{16}A$ present in the element.

• A. 90 %
• B. 60 %
• C. 45 %
• D. 80 %

Answer 1

The correct option is A 90 %

Let %age composition of $A_8^{16}$ $=x$ ; then %age composition of $A_8^{18}$ $=100-x$

Applying the formula for relative atomic mass as; $16.2=\frac{[x\times 16]+\left[\right(100-x)\times 18]}{100}$
$1620=16x+1800-18x$

$x=90$%

Therefore, the percentages of $A_8^{16}$ and $A_8^{18}$ are 90% and 10% respectively.