Which of the following could lead to a yield lower than 100%?

A yield lower than 100% can occur due to side reactions. During a chemical reaction, the reactants may not only produce the desired product but also unintended products through processes known as side reactions. These side reactions consume some of the starting materials, reducing the amount of the desired product obtained.

For instance, in a reaction involving multiple reactants, if one of the reactants can react with other substances present in the reaction mixture, this can lead to the formation of by-products. Consequently, the overall yield of the desired product decreases because some reactants are diverted to create these by-products instead.

While excess reagents typically do not affect the yield negatively (as they ensure that the limiting reactant is fully consumed), catalyst errors do not directly reduce yields since they enhance reaction rates and are not consumed in the reaction. The mention of perfect reactions inaccurately implies a scenario where all reactants are converted to products without any losses, which is not realistic in practical chemistry. Thus, side reactions are the primary reason for yielding less than 100% of the desired product.