OCR A Level Chemistry A (H432) Chemistry B (H433) PAG 6: Synthesis of an organic solid Practice Exam Questions and Mark Scheme

This resource includes exam questions from both Chemistry A and Chemistry B. It is not designed as a topic test but is instead intended as a document to support the teaching and learning of Module 1 – Development of practical skills in chemistry. 1. A solid organic compound can be purified by recrystallisation. Which statement(s) about recrystallisation is/are true? 1 The organic compound is more soluble in hot solvent. 2 The hot solution is cooled before the purified organic compound is collected. 3 The melting point of the purified organic compound is lower than the impure compound. A 1, 2 and 3 B Only 1 and 2 C Only 2 and 3 D Only 1 Your answer [1] 2. When are insoluble impurities removed during recrystallisation? A when the hot solution is filtered B as the solution cools C when the crystals are filtered off D when the crystals are washed Your answer [1] 2(a). Benzoic acid, C6H5COOH, is added to some foods as a preservative. A student prepares benzoic acid as outlined below. The student mixes 4.00 cm3 of phenylmethanol, C6H5CH2OH, (density = 1.04 g cm−3) with Step 1 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 . Impure crystals of benzoic acid appear. Step 3 The student recrystallises the impure crystals to obtain 1.59 g of pure benzoic acid. In Step 1, sodium carbonate, Na2CO3, makes the reaction mixture alkaline. Write an ionic equation to show how carbonate ions form an alkaline solution in water. [1] (b). In Step 2, explain why the mixture must be acidified so that crystals of benzoic acid appear. [1] (c). Write the overall equation for the preparation of benzoic acid from phenylmethanol. Use [O] for the oxidising agent. [1] (d). Calculate the percentage yield of benzoic acid. Give your answer to 3 significant figures. percentage yield = % [3] (e). In Step 3, describe how the student can recrystallise the impure crystals to obtain pure benzoic acid. [2] 3(a). * Aspirin can be made by refluxing 2-hydroxybenzoic acid with ethanoic anhydride, (CH3CO)2O, in the presence of a concentrated acid catalyst. • Aspirin is much more soluble in hot water than in cold water. • The percentage yield of aspirin from this method is 90.0%. Outline a plan to prepare 8.10 g of pure aspirin. In your answer, include the mass of 2-hydroxybenzoic acid needed and the purification steps from the hot reaction mixture. [6] (b). The Rf values and melting point ranges of 2-hydroxybenzoic acid and pure aspirin are shown in the table. Compound Rf Melting point range / °C 2-Hydroxybenzoic acid 0.30 158–161 Pure aspirin 0.75 138–140 i. A student analyses the purity of their impure aspirin by thin-layer chromatography (TLC). From the results the student concludes that the impure aspirin is contaminated with a small amount of unreacted 2-hydroxybenzoic acid. Draw spots on the chromatogram below to show how the student arrived at this conclusion.