Naturally occurring iron contains:

5.82% 54Fe (atomic mass = 53.940 amu)

91.66 % 56Fe (atomic mass = 55.935 amu)

2.19% 57Fe (atomic mass = 56.935 amu)

0.33% 58Fe (atomic mass = 57.933 amu)

Calculate the average atomic mass of iron.

The average atomic mass of iron is the sum of the products of the percentage abundance and masses of the isotopes. That is,

average atomic mass = 0.0582 x 53.940 amu + 0.9166 x 55.935 amu + 0.0219 x 56.935 amu + 0.0033 x 57.933 amu = 55.847 amu

Thus, the average atomic mass of iron is approximately 55.847 amu.