Post-Laboratory Questions 1. Determine the % yield of aspirin. 2. Compare the me
ID: 580557 • Letter: P
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?
Explanation / Answer
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.