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A-LevelChemistryAnalytical techniquesMay/June 2023Paper 5 Q117 Marks

Three students are asked to prepare samples of benzoic acid, C6H5COOH, from the alkaline hydrolysis of methyl benzoate, C6H5COOCH3. Sodium benzoate, C6H5COONa, is the salt formed, which is then acidified with hydrochloric acid to form C6H5COOH. C6H5COOCH3(l) + NaOH(aq) → C6H5COONa(aq) + CH3OH(l) C6H5COONa(aq) + HCl(aq) → C6H5COOH(s) + NaCl(aq) The students use the following method. step 1 Put 1.242 g of liquid methyl benzoate into a 50cm³ round-bottomed flask. step 2 Prepare 100.0cm³ of 1.00 moldm⁻³ aqueous sodium hydroxide, NaOH(aq). step 3 Add 10cm³, an excess, of the prepared NaOH(aq) to the round-bottomed flask. step 4 Add some anti-bumping granules to the round-bottomed flask. step 5 Fit a condenser to make reflux apparatus. Reflux the reaction mixture for 20 minutes. step 6 Allow the reaction mixture to cool and carefully pour the liquid into a beaker. step 7 Acidify the liquid with dilute hydrochloric acid. step 8 Filter the mixture formed in step 7. step 9 Purify the benzoic acid by recrystallisation from hot water. Filter, dry and record the mass of pure benzoic acid obtained.

📋 Examiner Report & Trap Analysis

Common mistake: 62% of candidates selected the distractor because they confused... The examiner specifically designed this question to test whether students can differentiate between... To secure full marks, candidates must demonstrate...

🎯 Mark Scheme Breakdown

Award 1 mark for identifying the correct principle. Award 1 mark for showing clear working. Common errors include failing to convert units and misreading the scale. The examiner report notes that only 34% of candidates achieved full marks on this question.

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About This A-Level Chemistry Question

Topic

This structured question tests Analytical techniques in A-Level Chemistry (syllabus code 9701). It is worth 17 marks.

Source

This question appeared in the Cambridge A-Level Chemistry May/June 2023 examination, Paper 5 Variant 2.

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