Question

An element $\text{X}$ has two isotopes,${}^{56}\text{X}$ and ${}^{58}\text{X}$. If the natural abundance of lighter isotope is 20% than the average atomic mass of the element is:

• A. $56.8$
• B. $57.2$
• C. $57.4$
• D. $57.6$

Answer 1

Answer: (D)

$57.6$

Given Natural abundance of lighter element $=20$%

${\Rightarrow }_{56}X=20$%

$\therefore$ ${}_{58}X=80$%

Average atomic mass $=(56\times 0.2)+(58\times 0.8)=57.6$