Posted by Kerrie on .
How to calculate the theoretical yield of aspirin to be obtained when 2.0 g of salicylic acid and 5.0 mL of acetic anhydride (density = 1.08 g/mL) are mixed together? ( Molar masses: acetic anhydride = 102.1 g/mol; salicylic acid = 138.2 g/mol)
Most of these stoichiometry problems are worked the same way. This problem is a limiting reagent problem and stoichiometry rolled into one.
1. Write and balance the equation.
2. Convert g salicylic acid to moles. Moles = grams/molar mass. Convert 5.0 mL acetic anhydride (using density) to grams, then to moles.
3a. Using the coefficients in the balanced equation, convert moles salicylic acid to moles aspirin.
3b. Using the same procedure, convert moles acetic anhydride to moles aspirin.
3c. Most likely you will get two different answers for 3a and 3b. Obviously, both can't be correct. The correct answer in these problems is ALWAYS the smaller value.
4. Using the smaller value found in 3c, convert moles to grams of product. grams = moles x molar mass.
The answer obtained is the theoretical yield.