in 1 mole there are=6.02*10^23 particles
so there are 2 cl atoms
1.81*10^24 MgCl2 should containt 2*(1.81*10^24) atoms...2*1.8*10^24 = 3.6 * 10^24
as we knw 1 mole contains 6.02*10^23 particle
so x mole contains 3.6*10^24 particle
so approximately there are 6 moles of chlorine atoms
hope it helps