You're dealing with a synthesis reaction in which aluminium metal,
The balanced chemical equation that describes this reaction looks like this
#2"Al"_ ((s)) + color(red)(3)"Cl"_ (2(g)) -> color(blue)(2)"AlCl"_(3(s))#
Notice that you have a
In other words, the reaction will consume
The problem tells you that the reaction produced
#3 color(red)(cancel(color(black)("moles AlCl"_3))) * (color(red)(3)color(white)(a)"moles Cl"_2)/(color(blue)(2)color(red)(cancel(color(black)("moles AlCl"_3)))) = color(green)(|bar(ul(color(white)(a/a)"4.5 moles Cl"_2color(white)(a/a)|)))#
As a conclusion, the stoichiometric coefficients listed in the balanced chemical equation can be used as mole ratios between the chemical species that take part in a given reaction.