Science, Tech, Math › Science Theoretical Yield Worked Problem Amount of Reactant Needed to Produce a Product Share Flipboard Email Print Steve McAlister, Getty Images Science Chemistry Basics Chemical Laws Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Todd Helmenstine Todd Helmenstine is a science writer and illustrator who has taught physics and math at the college level. He holds bachelor's degrees in both physics and mathematics. our editorial process Todd Helmenstine Updated September 07, 2019 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 = 766.67 Answer 766.67 grams of salicylic acid are needed to produce 1000 1-gram aspirin tablets.