To convert from formula units to moles, you need to divide by Avogardo's constant. And then remember that there will be 2 moles of Cl for every 1 mole of MgCl₂.
1.81*10^24 formula units MgCl₂ * (1 mole MgCl₂/6.022*10^23 formula units MgCl₂) * (2 moles Cl/1 mole MgCl₂) = 6.01 moles Cl
There are 6.01 moles of Cl atoms in 1.81*10^24 formula units of MgCl₂.
