Question

Given that the abundance of isotopes ${}^{54}Fe{,}^{56}Fe,$ and ${}^{57}Fe$ is 5%, 90% and 5%, respectively. The atomic mass of Fe is
(IIT-JEE, 2009)

• A. 55.85
• B. 55.95
• C. 55.75
• D. 55.05

Answer 1

The correct option is B 55.95

The atomic mass of iron is the average of all the masses. So I take the percentage abundance of each multiplied by its molecular weight, and divide it by 100. Which is:
$=\frac{(5\times 54)+(90\times 56)+(5\times 57)}{100}=55.95$