Edexcel Jan 2015 (IAL) Paper 6 Q4 (with explained solutions) |

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Edexcel Jan 2015 (IAL) Paper 6 Q4

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4 Cholesteryl benzoate was the first liquid crystal to be discovered. It can be prepared by the following procedure. Step 1 Dissolve 1.0 g of cholesterol in 3 cm3 of pyridine in a conical flask. Step 2 Add 0.40 cm3 of benzoyl chloride. Step 3 Heat the mixture on a steam bath for about 10 minutes. Step 4 Cool the mixture, and add 15 cm3 of methanol. Step 5 Collect the solid cholesteryl benzoate by suction filtration. Rinse the flask and the crude crystals with a little cold methanol. Step 6 Recrystallize the cholesteryl benzoate using ethyl ethanoate as the solvent. Some physical data for the chemicals involved are shown below. Molar mass / g mol1 Density / g cm3 Melting Boiling temperature temperature / K / K Cholesterol Benzoyl chloride Cholesteryl benzoate Pyridine Ethyl ethanoate 386.7 140.6 490.8 79.1 88.1 1.21 423 190 (a) Suggest the apparatus you would use to measure the volume of benzoyl chloride. 633 470 388 350 (1)(b) The warning symbols on a bottle of benzoyl chloride are shown below. Write the meaning of each symbol in the space provided. (2)*P45046A01116* 11 Turn over<br />
(c) 1 mol of cholesterol reacts with 1 mol of benzoyl chloride to form 1 mol of cholesteryl benzoate. (i) Determine which reactant is in excess by calculating how many moles of cholesterol and of benzoyl chloride are used in the preparation. (ii) Calculate the percentage yield when 0.65 g of cholesteryl benzoate is obtained. (d) Suggest how the mixture is cooled in Step 4. (3) (2) (1)(e) Suggest why methanol is added to the cooled mixture in Step 4. (1)12 *P45046A01216*<br />
(f) Describe how to carry out the recrystallization to obtain pure dry crystals of cholesteryl benzoate in Step 6. (5)(g) How would you show that the recrystallized cholesteryl benzoate crystals in Step 6 are purer than the crude crystals obtained in Step 5? (2)(Total for Question 4 = 17 marks) TOTAL FOR PAPER = 50 MARKS *P45046A01316* 13<br />

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Question Number 4(a) Question Number 4(b) Question Number 4(c)(i) Acceptable Answers Reject Mark Measuring cylinder Syringe / (graduated) pipette / burette / micro-pipette / auto pipette / any pipette with a volume of 5cm3 or less ALLOW Calculate the mass and use a balance Recognisable spelling of pipette / burette but not biuret. Acceptable Answers Reject Mark Corrosive/causes burns/irritant/dangerous Oxidising/toxic/poisonous /irritant Toxic/poisonous (1) Ignore harmful Corrosive/causes burns (1) Ignore harmful Acceptable Answers Penalise rounding errors once only in parts (i) and (ii) Number of moles of cholesterol = 1.0 = 2.58598 x 10-3 (1) 386.7 = 2.59 x 10-3 Note 2.58 x 10-3 loses this mark as a rounding error but 2.6 x 10-3 is fine Mass of benzoyl chloride = 0.4 x 1.21 = 0.484 (g) (1) Number of moles of benzoyl chloride = 0.484 = 3.4424 x 10-3 140.6 AND So benzoyl chloride is in excess / cholesterol is the limiting factor (1) Reject Mark WCH0601 1501<br />
Question Number 4(c)(ii) Question Number 4(d) Question Number 4(e) Mark Reject 50.9% Acceptable Answers Number of moles of cholesteryl benzoate = 0.65 490.8 = 1.3244 x 10-3 (1) Yield = 1.32 x 10-3 x 100 2.59 x 10-3 = 51(.1)% Or = 51(.2) ignore SF (1) OR Expected/maximum mass of cholesteryl benzoate = 2.59 x 10-3 x 490.8 = 1.27 g (1) Yield = 0.65 1.27 = 51(.1)% Or = 51(.2) ignore SF (1) Correct answer no working (2) Acceptable Answers Reject Place flask in running cold water OR (in an) ice bath / beaker of cold / cool water Add ice / Put in the fridge Washing with / adding cold water / ice Acceptable Answers Reject To react with / remove (any residual) benzoyl chloride Acts as a solvent Removes impurities Mark Mark WCH0601 1501<br />
Question Number 4(f) Mark Acceptable Answers Dissolve / add / put crystals in minimum (volume / amount) (1) of hot ethyl ethanoate (1) Penalise incorrect solvent e.g. water once only. Filter (hot) and allow to cool (1) Filter and wash with small amount of / cold solvent. Note notice if solvent used here and in second marking point, this mark can be given (1) Note If ethyl ethanoate mentioned here can score second mark above and The wrong solvent may be used here e.g. water with the correct initial solvent. This should be separately penalised here. Dry between filter papers / dry in a desiccator ALLOW Keep suction filtration going until crystals are dry Dry in warm oven or below 100 C (1) Reject Wash to remove soluble impurities cold solution to remove insoluble impurities but only penalise soluble / insoluble once Dry in oven alone WCH0601 1501<br />
Question Number 4(g) Reject Mark Acceptable Answers Method 1 First Mark (The crystals from) step 6 / pure crystals have a sharp (at 150oC / 423 K) melting temperature / melt over 1-2oC OR (The crystals from) step 6 / pure crystals melt within 2oC of / close to the data book value / at 150oC / 423 K (1) Second Mark (The crystals from) step 5 would melt over a larger temperature range OR Would melt (significantly) more / more than 2oC and below the data book value / 150oC / 423 K (1) Method 2 Comparative answers Can score both marks if it clear which step the crystals are prepared in. Two Examples Both melting points measured and the pure sample / step 6 has a sharper melting temperature (2) Both melting points measured and the pure sample / step 6 has a melting temperature clos(er) to than data book value / 150oC / 423 K (1) The impure sample melts at a temperature below that of the pure sample (1) Allow reverse arguments for step 5 Total for Question 4 = 17 marks Total for paper = 50 marks WCH0601 1501<br />

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