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.
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