In one mol we can have 6.02*10^23 atoms or formula units, now to calculate how many moles we have in 4.45*10^24, we are able to do that using a simple math:
6.02 * 10^23 = 1 mol
4.45 * 10^24 = x moles
4.45 * 10^24/6.02 * 10^23x
x = 7.39 moles of FeCO3
