Assume you used 0.200 g of salicylic acid and 0.480 mL of acetic anhydride, and you obtain 0.152 g of pure, recrystallized, dry aspirin.

Question

Calculate the percent yield, clearly showing the calculations.

Answer 1

You must first write the correctly balanced chemical equation.

salicylic acid + acetic anhydride ==> aspirin + acetic acid

C₇H₆O₃ + C₄H₆O₃ ==> C₉H₈O₄ + C₂H₄O₂ 

In order to find the percent yield (actual yield/theoretical yield), we must first calculate the theoretical yield. This will depend on which reactant is limiting, ie is it salicylic acid or acetic anhydride.  Unfortunately, without knowing the density of the acetic anhydride, we cannot determine moles of acetic anhydride.  

 

If we assume that acetic anhydride is in excess, then the theoretical yield will depend only on moles of salicylic acid.

moles salicylic acid = 0.200 g/molar mass salicylic acid

 

Since 1 mole salicylic acid --> 1 mole aspirin, you now know the moles of aspirin

Grams of aspirin = moles aspirin x molar mass aspirin

 

Percent yield = (0.152 g/theoretical g of aspirin) x100% 

 

Answer should be 58.3% yield (to 3 significant figures)