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For part B of the problem it is asking "What mass of aluminum chloride can be produced by these reactants" and my chemistry tutor said we must use the limiting reactant to find the theoretical yield but I don't know why we can't use the excess reactant to find the yield.
The reactant that is present in lesser quantity, that is the limiting reagent determines the quantity of the product formed. This is because, the reaction would come to a stop once once one of the reactants is completely used up, hence it is important to know which reactant is in lesser quantity because that will determine when will the reaction will stop and how much of the product will be formed.
Basically we have to use the limiting reactant because the extent of the amount of product formed is determined by the limiting reactant. It helps to think of it this way, if you're making trying to make 2 cakes that require 2 eggs each, but you only have 3 eggs, you can only make one cake. The maximum number of cakes you can make (theoretical yield) is determined by the eggs (limiting reactant). I hope this helps :)