Question

Calculate the theoretical yield of aspirin obtained when 2.0 g of salicylic acid and 5.0 mL of acetic anhydride (density = 1.08 g/mL) are mixed together.(Molar masses: acetic anhydride = 102.1 g/mol; salicylic acid = 138.2 g/mol)If 1.6 g of aspirin were obtained in this experiment, what would be the percentage yield?The melting point of the crude aspirin was reported as 126 °C. What does it indicate about the purity of the product?

Answer 1

Answer:

The % yield = 61.3 %

The aspirin is impure, since the melting point was too low

Explanation:

Step 1: Data given

Mass of salicylic acid = 2.0 grams

Volume of acetic anhydride = 5.0 mL ( Density = 1.08 g/mL)

Molar mass of acetic anhydride =102.1 g/mol

Molar mass of salicylic acid = 138.2 g/mol

Step 2: The balanced equation

C7H6O3 + C4H6O3 → C9H8O4 + CH3COOH

Step 3: Calculate moles of salicylic acid

Moles salicylic acid = mass salicylic acid / Molar mass salicylic acid

Moles salicylic acid = 2.0 grams /138.2 g/mol

Moles salicylic acid = 0.0145 moles

Step 4: Calculate moles of acetic anhydride

Mass of acetic anhydride = 1.08 g/mL * 5.0 mL = 5.4 grams

Moles acetic anhydride = 5.4 grams / 102.1 g/mol

Moles acetic anhydride = 0.0529 moles

Step 5: Calculate limting reactant

Mole ratio is 1:1 so since salicylic acid has the smallest number of moles, it's the limiting reactant

Salicylic acid will completely be consumed (0.0145 moles)

Acetic anhydride is in excess. There will 0.0145 moles be consumed.

There will remain 0.0529 -0.0145 = 0.0384 moles of acetic anhydride

Step 6: Calculate moles of aspirin

Since mole ratio is 1:1 there will be produced 0.0145 moles of aspirin

Step 7: Calculate mass of aspirin

Mass of aspirin = 0.0145 moles * 180.158 g/mol

Mass of aspirin =  2.61 grams = theoretical yield

Step 8:  Calculate % yield

% yield = actual yield / theoretical yield

% yield = (1.6 grams / 2.61 grams)*100%

% yield = 61.3 %

Since the melting point of pure aspirin is 138-140, this means that the aspirin was impure. It's possible that the aspirin still has insoluble impurities in it.