We have the following reaction:
[tex]2Al+3Cl_2\rightarrow2AlCl_3[/tex]So as we can see for every 3 moles of Cl2 we get 2 moles of AlCl3. So to calculate how many moles of AlCl3 we get with 5.9 moles of Cl2 we do the following:
[tex]moles_{AlCl3}=5.9\text{ }mol_{Cl2}\text{ }x\text{ }\frac{2\text{ }mol_{AlCl3}}{3\text{ }mol_{Cl2}}=3.933\text{ }mol_{AlCl3}[/tex]So the answer is 3.933 moles of AlCl3
