Question

Aspirin can be made in the laboratory by reacting acetic anhydride (C 4H 6O 3) with salicylic acid (C 7H 6O 3) to form aspirin (C 9H 8O 4) and acetic acid (C 2H 4O 2). The balanced equation is
C 4H 6O 3+C 7H 6O 3?C 9H 8O 4+C 2H 4O 2
In a laboratory synthesis, a student begins with 2.80mL of acetic anhydride (density=1.08g/ml) and 1.24g of salicylic acid. Once the reaction is complete, the student collects 1.24g of aspirin.
Determine the limiting reactant for the reaction.
Determine the theoretical yield of aspirin for the reaction.
Determine the percent yield of aspirin for the reaction.

Answer 1

Answer: The percent yield of the aspirin is 77.5 %.

Explanation:

To calculate the mass of acetic anhydride, we use the equation:

$\text{Density of substance}=\frac{\text{Mass of substance}}{\text{Volume of substance}}$

Density of acetic anhydride = 1.08 g/mL

Volume of acetic anhydride = 2.80 mL

Putting values in above equation, we get:

$1.08g/mL=\frac{\text{Mass of acetic anhydride}}{2.80mL}\\\\\text{Mass of acetic anhydride}=(1.08g/mL\times 2.80mL)=3.024g$

To calculate the number of moles, we use the equation:

$\text{Number of moles}=\frac{\text{Given mass}}{\text{Molar mass}}$     .....(1)

• For acetic anhydride:

Given mass of acetic anhydride = 3.024 g

Molar mass of acetic anhydride = 102.1 g/mol

Putting values in equation 1, we get:

$\text{Moles of acetic anhydride}=\frac{3.024g}{102.1g/mol}=0.0296mol$

• For salicylic acid:

Given mass of salicylic acid = 1.24 g

Molar mass of salicylic acid = 138.12 g/mol

Putting values in equation 1, we get:

$\text{Moles of salicylic acid}=\frac{1.24g}{138.12g/mol}=0.0089mol$

The chemical equation for the combustion of hexane follows:

$C_4H_6O_3+C_7H_6O_3\rightarrow C_9H_8O_4+C_2H_4O_2$

By Stoichiometry of the reaction:

1 mole of salicylic acid reacts with 1 mole of acetic anhydride

So, 0.0089 moles of salicylic acid will react with = $\frac{1}{1}\times 0.0089=0.0089mol$ of acetic anhydride

As, given amount of acetic anhydride is more than the required amount. So, it is considered as an excess reagent.

Thus, salicylic acid is considered as a limiting reagent because it limits the formation of product.

By Stoichiometry of the reaction:

1 mole of salicylic acid produces 1 mole of aspirin

So, 0.0089 moles of salicylic acid will produce = $\frac{1}{1}\times 0.0089=0.0089moles$ of aspirin

Now, calculating the mass of aspirin from equation 1, we get:

Molar mass of aspirin = 180.16 g/mol

Moles of aspirin = 0.0089 moles

Putting values in equation 1, we get:

$0.0089mol=\frac{\text{Mass of aspirin}}{180.16g/mol}\\\\\text{Mass of aspirin}=(0.0089mol\times 180.16g/mol)=1.60g$

• To calculate the percentage yield of aspirin, we use the equation:

$\%\text{ yield}=\frac{\text{Experimental yield}}{\text{Theoretical yield}}\times 100$

Experimental yield of aspirin = 1.24 g

Theoretical yield of aspirin = 1.60 g

Putting values in above equation, we get:

$\%\text{ yield of aspirin}=\frac{1.24g}{1.60g}\times 100\\\\\% \text{yield of aspirin}=77.5\%$

Hence, the percent yield of the aspirin is 77.5 %.