NDA 1 2024 GAT, question 64

Topic: Chemistry

Q64

The relative atomic mass of boron (which exists in two isotopic forms $^{10}B$ and $^{11}B$ ) is $10.81$ . What will be the abundance of $^{10}B$ and $^{11}B$ , respectively (consider a sample of $100$ atoms) ?

(a) $19\%$ and $81\%$ Correct answer
(b) $81\%$ and $19\%$
(c) $38\%$ and $62\%$
(d) $62\%$ and $38\%$

Solution.

Answer: (a) $19\%$ and $81\%$

Let $x$ be the fraction of $^{10}B$ . Then $10x + 11(1 - x) = 10.81$ .
So $11 - x = 10.81$ and $x = 0.19$ .
$^{10}B$ is 19% and $^{11}B$ is 81%.

Trap: (b) gives the same two numbers in the reverse order; the average mass 10.81 is closer to 11, so $^{11}B$ must be the larger share.