A sample of aspirin was prepared by reacting 2.20 g of salicylic acid with 4.20 mL of ethanolic anhydride in a conical flask - VCE - SSCE Chemistry - Question 2 - 2009 - Paper 1

To find the initial amount in moles of ethanolic anhydride, first calculate its mass using the density formula:

$$ext{mass} = ext{volume} imes ext{density}$$

Given:

• Volume of ethanolic anhydride = 4.20 mL
• Density of ethanolic anhydride = 1.08 g mL⁻¹

Calculating the mass:

$$ext{mass} = 4.20 ext{ mL} imes 1.08 ext{ g mL}^{-1} = 4.536 ext{ g}$$

Now, calculate the moles:

ext{moles of ethanolic anhydride} = rac{4.536 ext{ g}}{102 ext{ g mol}^{-1}} = 0.0445 ext{ mol}

To determine the maximum mass of aspirin produced, identify the limiting reagent.

From the calculations:

• Moles of salicylic acid = 0.0159 mol
• Moles of ethanolic anhydride = 0.0445 mol

The stoichiometry of the reaction implies a 1:1 ratio of salicylic acid to aspirin, meaning salicylic acid is the limiting reagent.

Now calculate the theoretical mass of aspirin:

$$ext{mass of aspirin} = ext{moles} imes ext{molar mass}$$

Given:

• Molar mass of aspirin = 180 g mol⁻¹,
• Moles of salicylic acid (limiting reagent) = 0.0159 mol

Calculating:

$$ext{mass of aspirin} = 0.0159 ext{ mol} imes 180 ext{ g mol}^{-1} = 2.862 ext{ g}$$

The percentage yield can be calculated using the formula:

ext{Percentage yield} = rac{ ext{actual yield}}{ ext{theoretical yield}} imes 100 ext{ }

Given:

• Actual yield of aspirin = 2.25 g
• Theoretical yield of aspirin = 2.862 g

Substituting these values:

ext{Percentage yield} = rac{2.25 ext{ g}}{2.862 ext{ g}} imes 100 \\ = 78.4 ext{ }