How many grams of commercial C2H4O2 (97% pure by mass) must be allowed to react with an excess of PCl3 to produce 50.0g of C2H3OCL if the reaction has a 70% yield?

3C2H4O2 + PCl = 3C2H3OCl + H3PO4

well, how many moles of C2H3OCl is 50 grams?

then you needed that number of moles, times (1/.97)(1/.70), then convert that to grams of C3H4O2,