WJEC Chemistry Practice Exam

In the context of chemical reactions and yields, a yield lower than 100% indicates that not all of the reactants were converted into the desired product. Each of the reasons provided in the other options explains a potential loss or inefficiency in the reaction or the recovery of the product.

Total product recovery refers to the complete retrieval of the product after the reaction has occurred — essentially the process of collecting and measuring the actual yield of the product obtained. If total product recovery is accounted for, it means that all the possible product formed has been successfully collected, which would not contribute to a yield lower than 100%.

In contrast, side reactions (where reactants produce unwanted products), incomplete reactions (where not all reactants are converted into products), and loss during purification (where some of the product is lost during the separation or cleaning processes) all contribute to a lesser yield. Therefore, since total product recovery implies that you have captured and accounted for all the expected product, it does not contribute to a yield lower than 100% and is thus not a reason for such a yield.