For the synthesis of aspirin, how do you calculate the percent yield if the reaction produces 4.70 grams of aspirin. The reaction, using molecular formulas is C7H6O3 + C4H6O3 --> C9H8O4 +...

For the synthesis of aspirin, how do you calculate the percent yield if the reaction produces 4.70 grams of aspirin. The reaction, using molecular formulas is C7H6O3 + C4H6O3 --> C9H8O4 + C2H4O2. The relevant molecular weights are 180 grams per mole for aspirin, and 138 grams per mole for salicylic acid. An excess of acetic anhydride is used in this preparation. The limiting reagent for this reaction will be salicylic acid.

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gsenviro | College Teacher | (Level 1) Educator Emeritus

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Aspirin (C9H8O4) is generated from salicyclic acid (C7H6O3) and acetic anhydride (C4H6O3) using the following synthesis reaction:

C7H6O3  + C4H6O3  -----> C9H8O4  +  C2H4O2

and since acetic anhydride is in excess, salicylic acid will be the limiting agent.

Using stoichiometry: 1 mole of salicylic acid generates 1 mole aspirin. 

So, 180 gm aspirin can be generated from 138 gm salicylic acid.

Using the information from your other question where 4 gm salicylic acid was available and the aspirin yield was 4 x 180/138 gm = 5.22 gm 

and in this case, 4.7 gm aspirin was generated, i.e. the reaction was not perfect (< 100% efficiency).

Theoretical yield = 5.22 gm

Actual yield = 4.7 gm

% yield = Actual yield/theoretical yield    x100 = 4.7 x 100/5.22 = 90.04%

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