Question

Given that the abundance of isotopes 54Fe,56Fe and 57Fe is 5%, 90% and 5%, respectively. The atomic mass of Fe is

• A. 55.25 u
• B. 55.95 u
• C. 55.75 u
• D. 56.95 u

Answer 1

The correct option is B 55.95 u

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