Question

There are two isotopes of an element with atomic mass $z$. Heavier one has atomic mass $z+2$ and lighter one has $z-1$, then an abundance of lighter one is:

• A. $66.6\%$
• B. $96.7\%$
• C. $6.67\%$
• D. $33.3\%$

Answer 1

Answer: (A)

$66.6\%$

Abundance of an isotope can be calculated as :

Let abundance of isotope with atomic mass $Z+2$ is x

Then abundance of isotope $Z-1$ is $1-x$

$(Z+2)x+(Z-1)(1-x)=Z$

On solving this, we get

$x=\frac{1}{3}$

Therefore,% abundance of isotope with atomic no.$Z+2$ is =$=\frac{1}{3}\times 100=33.33$%

Therefore,% abundance isotope with atomic no. $Z-1$ is =$100-33.33=67.66$%

Therefore isotope with atomic $Z-1$ is higher in % abundance