This answer is the amount of carbon dioxide produced if all of the ethane was converted to product. There is not enough oxygen to accomplish this. Try doing the same calculation starting with the 64 grams of oxygen and you will see that less product is possible. Remember that when arbitrary quantities of two reactants are given in the problem, the problem must be solved twice...for each reactant separately. Try doing this and see which of the two answers is the smaller number...that will be the correct answer.