The natural isotopes of Iron are Fe-55 and Fe-56. If the average atomic mass is 55.85, what are the percent abundances of each isotope?

15%

85%

Explanation:

Let the natural abundance of Fe-55  = y

      the natural abundance of Fe - 56  = 1-y

Average atomic mass = 55.85

Unknown:

Percent abundances of each isotope = ?

Solution;

The average atomic mass of a substance is given as;

  Average atomic mass  =  (mass x abundance)Fe - 55 + (mass x abundance)Fe - 56

Now insert the parameters and solve;

55.85  = (55 x y) + [56 x(1-y)]

 55.85  = 55y + 56 - 56y

 55.85  = -y + 56

  55.85 - 56  = -y

  -0.15 = -y

     y  = 0.15

1-y  = 1 - 0.15  = 0.85

Percentage abundance;

   Fe - 55  = 0.15 x 100  = 15%

   Fe - 56  = 0.85 x 100  = 85%

it is the most commonly accepted one