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

First, calculate the mass of ethanoic anhydride using its density:

$m = \text{volume} \times \text{density}$

Given:

• Volume = 4.20 mL
• Density = 1.08 g mL⁻¹

Calculating:

$m = 4.20 \, \text{mL} \times 1.08 \, \text{g mL}^{-1} = 4.536 \, \text{g}$

Now, find the moles:

$n = \frac{m}{M} = \frac{4.536 \, \text{g}}{102 \, \text{g mol}^{-1}} = 0.0445 \, \text{mol}$

Since salicylic acid is the limiting reagent, use the moles of salicylic acid to find the maximum mass of aspirin:

From the equation, 1 mol of salicylic acid produces 1 mol of aspirin:

Maximum moles of aspirin = moles of salicylic acid = 0.0159 mol

Now convert this to mass:

$m = n \times M = 0.0159 \, \text{mol} \times 180 \, \text{g mol}^{-1} = 2.862 \, \text{g}$

Percentage yield can be calculated using the formula:

$\text{Percentage yield} = \left( \frac{\text{mass of aspirin collected}}{\text{mass of aspirin theoretical}} \right) \times 100$

Substituting the values:

• Mass of aspirin collected = 2.25 g
• Mass of aspirin theoretical = 2.862 g

Calculating:

$\text{Percentage yield} = \left( \frac{2.25}{2.862} \right) \times 100 = 78.4 \%$