Question

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. What is the limiting reagent?

Answer 1

Given the following reaction C7H6O3 + C4H6O3 <---> C9H8O4 + C2H4O2

Molar mass of salicylic acid = 138.12 g/mol

Molar mass of acetic anhydride = 102.09 g/mol

Molar mass of acetylsalicylic acid = 180.157 g/mol

Given that we have 2g of salicylic acid, you have 2/138.12 = 0.014 mol

With the 5mL of acetic anhydride and its density, you can obtain the mass

density= mass/volume ---> (1.08g/mL)*(5mL)= 5.4 g of acetic anhydride

Given that you have 5.4g of acetic anhydride, you have 5.4g/102.09g/mol = 0.528 mol

Notice that the equation is a 1:1 ratio. That is 1 mole of salicylic acid gives 1 mole of aspirin.

Also, because there are less salicylic acid moles than acetic anhydride ones, the limiting reagent is the salicylic acid.

The theoretical yield can be obtained as follows:

2 g Salicylic acid* [1 mol of Salicylic acid/138.02g/mol]*[1 mol of aspirin/1 mol of salicylic acid][180.157g of aspirin/1 mol of aspirin]=2.608g of aspirin

Therefore, for 2 g of Salicylic acid you will get 2.608g of aspirin.