Question

Post-Laboratory Questions 1. Determine the % yield of aspirin. 2. Compare the melting point of the crude and the recrystallized aspirin with the theoretical melting point. a) Determine the % error b) Did the recrystallization process achieve any purification effect? c) Should the melting point of the recrystallized aspirin be higher or lower than that of the crude aspirin? 3. Discuss the FeCb test for impure and recrystallized aspirin. 4. 2.52 g of salicylic acids are reacted with 9.87 g of acetic anhydride. a) What mass of aspirin can be maximally produced? b) What mass is left of the excess reactant? c) If in the reaction above only 1.23 g of aspirin is obtained, what is the percent yield?

Answer 1

a) First write balanced chemical equation

C₄H₆O₃ + C₇H₆O₃ ------>C₉H₈O₄ + CH₃CO₂H

Acetic anhydride + salicylic acid ----> aspirin + acetic acid

We calculate the moles of each reactant

Molar mass of acetic anhydride = 102.089 g/mol

Mass of acetic anhydride = 9.87 g

Moles of acetic anhydride = Mass of acetic anhydride/ Molar mass of acetic anhydride

    = 9.87 g/102.089 g/mol

      = 0.0967 mol

Molar mass of salicylic acid = 138.121 g/mol

Mass of salicylic acid = 2.52 g

Moles of salicylic acid = Mass of salicylic acid / Molar mass of salicylic acid

                                        = 2.52 g/138.121 g/mol

           = 0.0182 mol

Acetic anhydride and salicylic acid reacted in 1:1 molar ratio and mol of salicylic acid is fewer than acetic anhydride, so salicylic acid is limiting reactant.

From molar ratios of the balanced equation to

Calculate the moles of aspirin, produced from salicylic acid

The balanced equation shows 1:1 molar ratio between salicylic acid and aspirin

So, moles of aspirin produced = moles of salicylic acid reacted = 0.0182 mol

Calculate the mass of aspirin by multiplying moles of aspirin with molar mass of aspirin

Molar mass of aspirin = 180.157 g/mol

Mass of aspirin = 0.0182 mol x 180.157 g/mol = 3.28 g

3.28 g of aspirin can be maximally produced.

b) Mass left of the excess reactant

Moles of excess reactant, which react = 0.0182 mol

Moles of excess reactant left = 0.0967 mol - 0.0182 mol = 0.0785 mol

Calculate the mass of excess reactant left by multiplying moles of excess reactant with its molar mass.

Mass of excess reactant left = 0.0785 mol x 102.089 g/mol = 8.01 g

c) In the reaction only 1.23 g of aspirin is obtained, this is actual yield.

Percent yield is the ratio of the actual yield to the theoretical yield.

Percent yield = [actual yield /theoretical yield] x 100

Percent yield = [1.23 g/3.28g] x 100

          = 37.5%

2. The melting point range of pure aspirin is 138-140 ^oC . If impurities are present in crude aspirin, the melting point range for product will be theoretically lower than the range of pure aspirin.

The experimental melting point may be 122 -132°C, due to impurity. The melting point of the recrystallized aspirin would be higher than crude aspirin.

If the Aspirin and solvent are not filtered before crystallization process starting, then impurities such as dirt and other contaminants could bond to the crystallizing Aspirin and melting point would be lower.