Question

Michael Farday weighs out 3.12 g. of salicylic acid in a 100 mL flask and adds 1.89 mL of acetic anhydride (density = 1.080 g/mL). Determine the maximum (theoretical) number of grams of acetylsalicylic acid he can prepare. If Mr. Faraday obtained 3.38 g. of Aspirin, calculate the % yield. Briefly, explain why he might have obtained this amount of product.

Answer 1

       C₇H₆O₃ +      C₄H₆O₃      C₉H₈O₄          +       C₂H₄O₂

salicylic acid    acetic anhydride acetylsalicylic acid     acetic acid

Mass of salicylic acid is = 3.12 g

Mass of acetic anhydride is , m = volume x density

                                               = 1.89 mL x 1.080 g/mL

                                               = 2.04 g

Molar mass of acetic anhydride is 102 g/mol

Molar mass of salicylic acid is 138 g/mol

MOlar mass of acetylsalicylic acid is 180 g/mol

According to the balanced equation,

1 mole =138 g of salicylic acid reacts with 1 mole=102 g of acetic anhydride

M g of salicylic acid reacts with 2.04 g of acetic anhydride

M = ( 2.04x138)/102

   = 2.76 g

So 3.12-2.76 = 0.36 g of salicylic acid left unreacted so it is the excess reactant.

Since all the mass of acetic anhydride reacted completly it is the limiting reactant.

So 1 mole = 102 g of acetic anhydride produces 1mole=180 g of aspirin

2.04 g of acetic anhydride produces N g of aspirin

N = ( 2.04x180)/102

   = 3.6 g   -------> This is the theoretical yield

Given actual mass of aspirin is 3.38 g

So percentage yield = ( actual mass / theoretical mass) x 100

                             = ( 3.38 / 3.6 ) x100

                            = 93.9 %