Aspirin can be produced by reacting salicylic acid with ethanolic anhydride - AQA - A-Level Chemistry - Question 4 - 2020 - Paper 2

A 50 cm³ measuring cylinder is suitable for measuring ethanolic anhydride as it provides accuracy and allows for easy pouring into the conical flask.

Concentrated phosphoric acid is corrosive and can cause skin burns or permanent eye damage if it comes into contact with skin or eyes.

The balanced equation is:

$ext{Salicylic acid (C}_7 ext{H}_6 ext{O}_3 ext{) } + ext{Ethanolic anhydride (C}_4 ext{H}_6 ext{O}_3 ext{) } \rightarrow ext{Aspirin (C}_9 ext{H}_8 ext{O}_4 ext{) } + ext{Acetic acid (C}_2 ext{H}_4 ext{O}_2$

Firstly, calculate the moles of salicylic acid and ethanolic anhydride:

• Moles of salicylic acid: ext{Moles} = rac{ ext{Mass}}{ ext{Molar Mass}} = rac{6.01 ext{ g}}{138.0 ext{ g/mol}} = 0.0436 ext{ mol}

• Moles of ethanolic anhydride:

Using density to find mass: $ext{Mass} = ext{Volume} imes ext{Density} = 10.5 ext{ cm}^3 imes 1.08 ext{ g/cm}^3 = 11.34 ext{ g}$

So, ext{Moles of ethanolic anhydride} = rac{11.34 ext{ g}}{102.0 ext{ g/mol}} = 0.111 ext{ mol}

The stoichiometry of the reaction shows that 1 mole of salicylic acid reacts with 1 mole of ethanolic anhydride. Since we have 0.0436 moles of salicylic acid and 0.111 moles of ethanolic anhydride, salicylic acid is the limiting reagent and therefore ethanolic anhydride is in excess.

Next, calculate the mass of aspirin produced:

• Theoretical yield of aspirin: $ext{Moles of aspirin produced} = ext{Moles of salicylic acid} = 0.0436 ext{ mol}$ $ext{Mass of aspirin} = ext{Moles} imes ext{Molar Mass} = 0.0436 ext{ mol} imes 180.0 ext{ g/mol} = 7.85 ext{ g}$

• Actual yield of aspirin: $ext{Mass produced} = ext{Theoretical Yield} imes ext{Yield Percentage} = 7.85 ext{ g} imes 0.841 = 6.60 ext{ g}$

Difference 1: The melting point of crude aspirin would likely be lower than that of pure aspirin due to the presence of impurities.

Difference 2: The melting point of crude aspirin may also show a broader melting point range compared to the sharp, well-defined melting point of pure aspirin.

Precaution 1: Use a water bath instead of a direct flame to prevent overheating and possible decomposition of the aspirin.

Precaution 2: Ensure that the apparatus is well-ventilated to safely disperse any vapors released during the heating process.

The purpose of adding a small amount of cold ethanol is to help precipitate the purified aspirin from the solution, facilitating the filtration process and enhancing the yield of the product.

One difference you would expect is that the crude aspirin would appear as larger, irregular crystals, while the purified aspirin would be smaller, needle-like crystals that are clearer in color.