Iron has an atomic weight of 56 which can be considered as 56 grams so one mole would be 56 grams. So 500 grams would be 8.92 moles. One mole has 6x10 to the 23rd atoms. So one gram would be 1/56 of the mole = 6x 10 to the 23rd/56= 1.1 x 10 to the 22nd atoms and then 500 x 1.1 x 10 to the 22nd atoms or 550 x 10 to the 22nd atoms.