March 26, 2012

The problem: I started with 2.0 grams of salicylic acid and 5 grams of acetic anhydride and I got 2.1 grams of acetylsalicylic acid (aspirin). What is my theoretical yield and my percent yield? The process to solve this should be as follows:

1.       Write balanced equation

2.       Calculate number of moles in 2.1 g of salicylic acid (SA)

3.       Calculate moles of acetic anhydride

4.       Based on the amounts of reagents specified in the experiment, calculate the theoretical yield of aspirin

5.       Solve actual divided by theoretical multiplied by 100 to get the percent yield.

C₇H₆O₃ (salicylic acid) + C₄H₆O₃ (Ethanoic anhydride) à C₉H₈O₄ (Aspirin) + C₂H₄O₂ (Acetic Acid)

1 mole of SA produces 1 mole of aspirin which gives this equation a 1:1 ratio.

To calculate the number of moles of aspirin, do the following calculation:

0.0152 mole SA  X  (1 mole aspirin/ 1 mole of SA) = 0.0152 mole aspirin

Then calculate the mole of aspirin to the number of grams of aspirin.

                0.0152 mole aspirin  X  (180 grams SA/ 1 mole aspirin) = 2.74 grams aspirin

The number of moles just calculated is the theoretical yield and now you can solve for the percent yield.

% Yield = (Actual/Theoretical) X 100

(2.1 grams Aspirin)/(2.74 grams Aspirin) X 100 = 76.6 %