Question

a) List two factors other than user error that would contribute to a yield greater than 100%. (2 marks)

b) List two factors other than user error that would contribute to a yield lower than 100%. (2 marks)

Context: a synthesis of aspirin lab, where we: "prepared a sample of aspirin, recrystallized the raw product, and determined the percentage yield of the aspirin based on the limiting reagent."

Answer 1

Ans a) Yield greater than 100% is difficult to theoretically not possible but if observed could be because of the following reasons:

i. The product has some amount of solvent left in it which increases its mass.

ii. The product is not worked up properly after the reaction, for example in case of aspirin synthesis, if unreacted salicylic acid is left in the reaction. Also, if during filtration impurities could not be removed completely, they could increase the final weight of compound.

Ans b ) A yield lower than 100% is generally observed in chemical reactions. The instances where 100% yield is obtained are very rare. This could be because of :

i) Formation of side products/ impurity in the reaction which consume some amount of the reactants.

ii) Some amount of product lost in the purification process like recrystallization in case aspirin synthesis.