Benzoic acid can be prepared from ethyl benzoate - AQA - A-Level Chemistry - Question 3 - 2017 - Paper 3

To calculate the amount of sodium hydroxide:

1. Calculate the amount of ethyl benzoate:

   Mass of ethyl benzoate = density × volume = 1.05 g cm⁻³ × 5.0 cm³ = 5.25 g

   Moles of ethyl benzoate = mass / molar mass = 5.25 g / 150 g mol⁻¹ = 0.0350 mol

2. Calculate moles of sodium hydroxide used:

   Volume of NaOH = 30.0 cm³ = 0.0300 L

   Concentration = 2 mol dm⁻³ = 2 mol / 1 dm³

   Moles of NaOH = concentration × volume = 2 mol dm⁻³ × 0.0300 L = 0.0600 mol

3. Determine excess sodium hydroxide:

   Since 0.0600 mol of NaOH is used and only 0.0350 mol is needed, there is excess NaOH present.

An excess of sodium hydroxide ensures that all of the ethyl benzoate reacts completely, preventing any unreacted starting material from remaining in the solution.

An electric heater provides a more controlled and uniform heating, reducing the risk of igniting flammable vapors and allowing for a consistent reaction temperature.

Reflux is used to ensure that any vaporized components condense and return to the reaction mixture, allowing the hydrolysis reaction to proceed efficiently without loss of reactants.

The reaction between sodium benzoate and hydrochloric acid can be represented as:

$\text{C}_6\text{H}_5\text{COO}^-\text{Na}^+ + \text{HCl} \rightarrow \text{C}_6\text{H}_5\text{COOH} + \text{NaCl}$

Sodium benzoate is soluble in cold water due to the ionic nature of the sodium ion, which can interact favorably with water molecules. In contrast, benzoic acid, being a neutral molecule, has limited solubility due to hydrophobic interactions of its hydrocarbon group.

To purify the benzoic acid, the student should:

1. Dissolve the crude benzoic acid in a minimum volume of hot water to form a solution.
2. Filter the solution while hot to remove any insoluble impurities.
3. Allow the solution to cool slowly, which will facilitate the crystallization of pure benzoic acid.
4. Collect the crystals by suction filtration.
5. Wash the crystals with cold water to remove any adhering impurities.
6. Finally, dry the purified benzoic acid using a drying oven or air dry.

To calculate the percentage yield:

1. Calculate the theoretical yield of benzoic acid from the amount of ethyl benzoate used: Given 0.0400 mol of ethyl benzoate produces 0.0400 mol of benzoic acid, which corresponds to:

   Mass of benzoic acid = moles × molar mass = 0.0400 mol × 122.12 g mol⁻¹ = 4.8848 g

2. Calculate the percentage yield:

   Percentage yield = (actual yield / theoretical yield) × 100 = (5.12 g / 4.8848 g) × 100 ≈ 104.81%

   However, since yield cannot exceed 100%, the yield is noted as 100%.

The yield may not be 100% due to several factors:

• Loss during the transfer of materials between containers,
• Incomplete reaction where some ethyl benzoate may not have reacted,
• Side reactions producing by-products,
• Loss of product during crystallization or filtration.