Edexcel Jun 2017 (IAL) Paper 3 Q4

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4 A student carried out an experiment to determine the concentration of ethanoic acid, CH3COOH, in a sample of vinegar. Procedure 1. A 25.0 cm3 sample of vinegar is transferred into a 250.0 cm3 volumetric flask using a pipette fitted with a pipette filler. This solution is made up to the mark with distilled water and mixed thoroughly. 2. A pipette is used to transfer 25.0 cm3 of the diluted solution into a conical flask. 3. A burette is filled with 0.100 mol dm3 sodium hydroxide solution. 4. The diluted vinegar solution is titrated with the sodium hydroxide solution, using phenolphthalein as an indicator. Results Number of titration Burette reading (final) / cm3 Burette reading (initial) / cm3 Volume of NaOH used / cm3 Used to calculate mean () 22.90 0.00 22.90 22.85 0.00 22.85 24.45 1.50 22.95 (a) (i) Show which titres are concordant by putting a tick () in the appropriate boxes in the table of results and hence calculate the mean titre. (ii) Calculate the number of moles of sodium hydroxide in the mean titre. (1) (1) *P48384A01520* 15 Turn over<br /> (iii) The equation for the reaction between ethanoic acid and sodium hydroxide is shown. CH3COOH(aq) + NaOH(aq)CH3COONa(aq) + H2O(l) Calculate the number of moles of ethanoic acid in the 25.0 cm3 sample of undiluted vinegar. Assume that no other acids are present in the vinegar. (iv) Calculate the concentration, in mol dm3, of the ethanoic acid in the sample of undiluted vinegar. (v) Calculate the concentration, in g dm3, of the ethanoic acid in the sample of undiluted vinegar. Give your answer to three significant figures. (2) (1) (3) 16 *P48384A01620*<br /> (b) The pipette is rinsed out with the diluted vinegar just before use. Explain why the pipette is rinsed. Comment on why it is rinsed with diluted vinegar rather than distilled water. (2)(c) When making up the diluted solution of vinegar, another student noticed some vinegar remaining in the tip of the pipette. This student used the pipette filler to blow this vinegar into the volumetric flask before making up to the mark. State and explain the effect, if any, this would have on the mean titre in the experiment. (2)(Total for Question 4 = 12 marks) TOTAL FOR PAPER = 50 MARKS *P48384A01720* 17<br />

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