1) Convert the number of molecules into the number of Cl atoms
[tex]Cl\text{ atoms=1.81}\cdot10^{24}moleculesofMgCl_2\cdot\frac{2\text{ atoms of Cl}}{1moleculeofMgCl_2}=3.62\cdot10^{24}\text{ Cl atoms}[/tex]There are 3.62*10^24 Cl atoms in 1.81×10^24 of MgCl2.
2) Convert the number of Cl atoms into moles of Cl.
[tex]\text{mol Cl=3.62}\cdot10^{24}\text{ Cl atoms}\cdot\frac{1\text{ mol Cl}}{6.022\cdot10^{23}\text{ Cl atoms}}=6.011\text{ mol Cl.}[/tex]There is 6.011 mol Cl in 1.81×10^24 of MgCl2.