BS Chemistry Lab: Aspirin Yield and Limiting Reagent
A BS candidate prepares acetylsalicylic acid from 5.00 g salicylic acid (MW 138.12) and 8.0 mL acetic anhydride (density 1.08 g mL⁻¹, MW 102.09) with three drops of sulfuric acid. After recrystallization from ethanol–water, 4.10 g of dry product is obtained (MW 180.16). The partner uses 6.00 g salicylic acid and the same volume of acetic anhydride and isolates 4.80 g. Both products melt sharply at 135–136 °C. The candidate suspects that part of the acetic anhydride hydrolysed during storage, lowering the yield. The stock bottle was opened repeatedly over a semester. The candidate must judge whether hydrolysis, recrystallization loss, or an error in the limiting-reagent calculation best explains the two results.
1. Identify the limiting reagent for the candidate's run and calculate the theoretical yield. Does the partner's run change the limiting reagent?
Read the worked answer
Salicylic acid: n = 5.00 / 138.12 = 0.03620 mol. Acetic anhydride: mass = 8.0 × 1.08 = 8.64 g, n = 8.64 / 102.09 = 0.08464 mol.
The anhydride is in 2.34-fold excess, so salicylic acid is limiting. Theoretical aspirin = 0.03620 × 180.16 = 6.52 g. Percent yield = 4.10 / 6.52 = 62.9%.
For the partner, n_sal = 6.00 / 138.12 = 0.04344 mol; anhydride is still 0.08464 mol, ratio 1.95, so salicylic acid is still limiting. Theoretical = 7.83 g, yield = 4.80 / 7.83 = 61.3%. The two yields are nearly the same percentage.
2. Quantify the hydrolysis hypothesis. What fraction of the acetic anhydride would have to hydrolyse before it becomes limiting? Would 20% hydrolysis explain the observed yield?
Read the worked answer
Let f be the fraction hydrolysed. Remaining anhydride = 0.08464(1 − f). For it to become limiting, set this equal to 0.03620:
0.08464(1 − f) = 0.03620 → f = 1 − 0.03620 / 0.08464 = 0.572.
So 57% of the anhydride would have to hydrolyse before the limiting reagent changes. At 20% hydrolysis, 0.0677 mol remains, still 1.87 times the salicylic acid; the theoretical yield is unchanged. Therefore hydrolysis cannot explain a yield near 63%. The partner's similar yield points to a systematic loss, most likely recrystallization.
3. Propose a simple diagnostic sequence that would separate recrystallization loss from incomplete reaction, using melting point, mass before recrystallization, and a FeCl₃ test. Predict the outcome if recrystallization is the main loss.
Read the worked answer
Weigh the crude product before recrystallization. If the crude mass is close to 6.5 g but the recrystallized mass is 4.1 g, the loss is in recrystallization. If the crude mass is already low, the reaction is incomplete.
A FeCl₃ test on the crude product: salicylic acid gives a purple complex, aspirin does not. A strong purple colour indicates unreacted salicylic acid; little or no colour indicates the reaction went to completion. Sharp melting point 135–136 °C confirms purity of the final product but says nothing about the yield.
Predicted outcome for recrystallization loss: high crude mass, weak FeCl₃ test, lower recrystallized mass.