Question

If you use 12.22 g of salicylic acid and an excess of acetic anhydride in the synthesis of aspirin, what would be the theoretical yield of acetylsalicylic acid in moles? In grams? Determine the percent yield if 10.50g of aspirin is isolated

Answer 1

Salicylic acid + acetic anhydride $\rightarrow$ aspirin + acetic acid

C₇H₆O₃   + C₄H₆O₃ $\rightarrow$    C₉H₈O₄ + CH₃COOH

Molar mass of salicylic acid,C₇H₆O₃   is = (7xAt.mass of C ) + ( 6xAt.mass of H ) + (3xAt.mass of O )

                                                                = (7x12) + (6x1) + (3x16)

                                                                = 138 g/mol

Given mass of C₇H₆O₃   is , m = 12.22 g

So number of moles of C₇H₆O₃    , n = mass/molar mass

                                                            = 12.22 g / 138 (g/mol)

                                                            = 0.089 mol

Since we are using excess of acetic anhydride it is the excess reactant.

and all the mass of salicylic acid is completly utilized , salicylic acid is the limiting reactant.

According to the balanced equation ,

1 mole of salicylic acid produces 1 mole of aspirin

0.089 mole of salicylic acid produces 0.089mole of aspirin

So the number of moles of aspin produced is 0.089 mol

Molar mass of aspirin ,    C₉H₈O₄ is = (9xAt.mass of C ) + ( 8xAt.mass of H ) + (4xAt.mass of O )

                                                          = (9x12) + (8x1) + (4x16)

                                                          = 180 g/mol

So mass of aspirin produced , m = number of moles x molar mass

                                                    = 0.089 mol x 180 (g/mol)

                                                    = 16.02 g   ------> This is the theoretical yield

percent yield = ( actual yield / Theoretical yield) x 100

                     = (10.50 g / 16.02 g ) x 100

                     = 65.5 %