If you have 100.0 grams of carbon and 100.0 grams or iron, which sample will contain more atoms

First you need to solve for the number of moles in each.

1 mole of anything = atomic mas of that element (g)

Carbon has an atomic mass of 12.0107

1 mole of Carbon = 12.0107 g

Iron has an atomic mass of 55.845

1 mole of Iron = 55.845 g

now convert each from grams to moles

(grams of Carbon cancel and you are left with moles of carbon)

100.0 g Carbon x 1 mol Carbon = 8.326 mol Carbon

12.0107 g Carbon

(grams of Iron will cancel and you are left with moles of iron)

100.0 g Iron x 1 mol Iron = 1.791 mol Iron

55.845 g Iron

now we can use avogadro's number to solve for the amount of atoms in each

1 mol = 6.02x10^23 atoms

8.326 mol Carbonx 6.02 x 10^23 atoms = 5.012 x 10^24 atoms Carbon

1 mol Carbon

1.791 mol Iron x 6.02 x 10^23 atoms = 1.078 x 10^24 atoms Iron

1 mol Iron

5.012 x 10^24 atoms Carbon > 1.078 x 10^24 atoms Iron