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OCR A Level Chemistry A (H432) Chemistry B (H433)-With marking Scheme

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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 c ... ooled 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] Created in ExamBuilder 1 © OCR 2020 OCR A Level Chemistry A (H432) Chemistry B (H433) PAG 6: Synthesis of an organic solid Practice Exam Questions and Mark Scheme 2(a). Benzoic acid, C6H5COOH, is added to some foods as a preservative. A student prepares benzoic acid as outlined below. Step 1 The student mixes 4.00 cm3 of phenylmethanol, C6H5CH2OH, (density = 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 . 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. [Show More]

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