Aspirin (C9H8O4) can be prepared by the reaction of salicylic acid (C7H6O3) with acetic anhydride (in
excess) in the presence of phosphoric acid as catalyst. Initially 5.1 g salicylic acid was used, after
purification; 5.10 g of solid (Aspirin) was collected.
(a) Calculate the theoretical yield. (6 marks)
(b) Calculate the percentage yield for aspirin. (4 marks)
The following equation is balanced as follow:
Salicylic Acid(C7H6O3) Acetic Anhydride Aspirin(C9H8O4) Acetic Acid
1: 1 :1 :1
To save time in typing let's call salicylic acid something like SA. The molar mass of SA is 138.1 but you should confirm that.
mols SA = grams/molar mass = 5.10/138 = about 0.037 but you should recalculate and add another significant figure.
Looking at the equation, 1 mol SA produces 1 mol aspirin so you will get 0.037 mols aspirin. g aspirin = mols aspirin x molar mass aspirin = ?. This is the theoretical yield (TY).
b. The actual yield (AY) is 5.10 grams. %yield = (AY/TY)*100 = ?
Post your work if you get stuck.
To calculate the theoretical yield of aspirin, we need to determine the limiting reactant and use the stoichiometry of the balanced equation.
(a) Calculate the theoretical yield:
1. Calculate the molar mass of salicylic acid (C7H6O3):
Mass of C = 7 * 12.01 g/mol = 84.07 g/mol
Mass of H = 6 * 1.008 g/mol = 6.048 g/mol
Mass of O = 3 * 16.00 g/mol = 48.00 g/mol
Molar mass of salicylic acid = 84.07 g/mol + 6.048 g/mol + 48.00 g/mol = 138.12 g/mol
2. Convert the mass of salicylic acid to moles:
Moles of salicylic acid = mass / molar mass = 5.1 g / 138.12 g/mol = 0.0369 mol
3. According to the balanced equation, the stoichiometric ratio between salicylic acid and aspirin is 1:1. Therefore, the moles of aspirin formed will be the same as the moles of salicylic acid used.
4. Convert the moles of aspirin to mass:
Mass of aspirin = moles of aspirin * molar mass of aspirin
Molar mass of aspirin = 9 * 12.01 g/mol + 8 * 1.008 g/mol + 4 * 16.00 g/mol = 180.16 g/mol
Mass of aspirin = 0.0369 mol * 180.16 g/mol = 6.65 g
Therefore, the theoretical yield of aspirin is 6.65 g.
(b) Calculate the percentage yield:
Percentage yield = (actual yield / theoretical yield) * 100
Given that the actual yield is 5.10 g:
Percentage yield = (5.10 g / 6.65 g) * 100 = 76.7%
Therefore, the percentage yield of aspirin is 76.7%.
To calculate the theoretical yield of aspirin, we need to determine the limiting reactant in the reaction. The limiting reactant is the reactant that is completely consumed and determines the maximum amount of product that can be formed.
Let's start by calculating the molar masses of the compounds involved:
Molar mass of salicylic acid (C7H6O3) = (7*12.01) + (6*1.01) + (3*16) = 138.12 g/mol
Molar mass of acetic anhydride = (4*12.01) + (6*1.01) + (2*16) = 102.09 g/mol
Molar mass of aspirin = (9*12.01) + (8*1.01) + (4*16) = 180.16 g/mol
Now, we can calculate the moles of each reactant:
Moles of salicylic acid = mass / molar mass = 5.1 g / 138.12 g/mol = 0.0370 mol
Moles of acetic anhydride = excess (infinite) since it is given in excess
Since the balanced equation shows a 1:1 stoichiometric ratio between salicylic acid and aspirin, and acetic anhydride is in excess, we can conclude that salicylic acid is the limiting reactant.
Now, we can calculate the theoretical yield of aspirin using the moles of salicylic acid:
Theoretical yield = moles of limiting reactant * molar mass of aspirin
= 0.0370 mol * 180.16 g/mol = 6.66 g
Therefore, the theoretical yield of aspirin is 6.66 g.
To calculate the percentage yield of aspirin, we need to compare the actual yield (5.10 g) to the theoretical yield (6.66 g).
Percentage yield = (actual yield / theoretical yield) * 100
= (5.10 g / 6.66 g) * 100 = 76.6%
Therefore, the percentage yield of aspirin is 76.6%.