Benzoic acid, C6H5COOH\text{C}_6\text{H}_5\text{COOH}C6H5COOH, is used as a food preservative and chemical intermediate.
A student prepares benzoic acid as outlined below.
Step 1 The student mixes 5.20 cm35.20\text{ cm}^35.20 cm3 of benzyl alcohol, C6H5CH2OH\text{C}_6\text{H}_5\text{CH}_2\text{OH}C6H5CH2OH (density =1.04 g cm−3= 1.04\text{ g cm}^{-3}=1.04 g cm−3), with sodium carbonate and aqueous potassium manganate(VII) as an oxidising agent. The mixture is heated under reflux.
Step 2 The resulting mixture is cooled and then acidified with concentrated HCl\text{HCl}HCl. Impure crystals of benzoic acid appear.
Step 3 The student recrystallises the impure crystals to obtain 2.60 g2.60\text{ g}2.60 g of pure benzoic acid.
In Step 1, sodium carbonate, Na2CO3\text{Na}_2\text{CO}_3Na2CO3, makes the reaction mixture alkaline. Write an ionic equation to show how carbonate ions form an alkaline solution in water.
In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear.
Write the overall equation for the preparation of benzoic acid from benzyl alcohol. Use [O][\text{O}][O] for the oxidising agent.
Calculate the percentage yield of benzoic acid. Give your answer to 3 significant figures.
In Step 3, describe how the student can recrystallise the impure crystals to obtain pure benzoic acid.
