in the beginning you are able to decrease fee the atom of silver and 25 atoms of lead. a million mole of Fe weighs fifty 5.847g so 0.3 moles weighs fifty 5.847 x 0.3 = sixteen.745g. a million mole of Hydrogen weighs a million.008g. So 0.3 mol Fe is the heaviest.
1/2 mole of lead atoms (atomic weight 205) would outweigh 2 moles of chlorine (atomic weight 35) by about 30% and would outweight one atom of gold or 25 atoms of mercury by about 10^21 %
the lead because of the avogadro's constant the atom ones are too small, from memory i think the molecular mass of chlorine is more than 4 times lighter than lead so it's lead.