Theoretical Yield Worked Example Problem

This example problem demonstrates how to calculate the amount of reactant needed to produce a product. Problem Aspirin is prepared from the reaction of salicylic acid (C7H6O3) and acetic anhydride (C4H6O3) to produce aspirin (C9H8O4) and acetic acid (HC2H3O2). The formula for this reaction is C7H6O3 C4H6O3 → C9H8O4 HC2H3O2 How many grams of salicylic acid are needed to make 1,000 1-gram tablets of aspirin? (Assume 100 percent yield.) Solution Step 1: Find the molar mass of aspirin and salicylic acid. From the periodic table: Molar Mass of C 12 gramsMolar Mass of H 1 grams Molar Mass of O 16 grams MMaspirin (9 x 12 grams) (8 x 1 grams) (4 x 16 grams)MMaspirin 108 grams 8 grams 64 gramsMMaspirin 180 grams MMsal (7 x 12 grams) (6 x 1 grams) (3 x 16 grams)MMsal 84 grams 6 grams 48 gramsMMsal 138 grams Step 2: Find the mole ratio between aspirin and salicylic acid. For every mole of aspirin produced, 1 mole of salicylic acid was needed. Therefore the mole ratio between the two is one. Step 3: Find the grams of salicylic acid needed. The path to solving this problem starts with the number of tablets. Combining this with the number of grams per tablet will give the number of grams of aspirin. Using the molar mass of aspirin, you get the number of moles of aspirin produced. Use this number and the mole ratio to find the number of moles of salicylic acid needed. Use the molar mass of salicylic acid to find the grams needed. Putting all this together: grams salicylic acid 1,000 tablets x 1 g aspirin/1 tablet x 1 mol aspirin/180 g of aspirin x 1 mol sal/1 mol aspirin x 138 g of sal/1 mol sal grams salicylic acid 766.67Â   Answer 766.67 grams of salicylic acid are needed to produce 1000 1-gram aspirin tablets.