and someone said before, that Uranium one with 0.5kg, what did you have to do, i had no idea.
I'm not sure if it's right but I worked out how many moles of 0.5kg of uranium-235 is. So:
1 mole = 235g of uranium-235
therefore:
500g of uranium-235 = (1/235) x 500 = 2.13 moles.
From there multiply the number of moles by avogadro's constant to get the number of atoms in 500g of uranium-235. Multiply this by your difference in mass.