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Edexcel Jun 2010 Paper 4 Q20
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![20 Vinegar is used as a food preservative. It is an acidic solution containing ethanoic acid, CH3COOH. (a) A titration was carried out to measure the concentration of ethanoic acid in a sample of vinegar. 25.0 cm3 of a vinegar solution was titrated with a solution of sodium hydroxide, concentration 0.250 mol dm3. The concentration of the ethanoic acid in the vinegar solution was found to be 0.125 mol dm3. (i) Calculate the pH of 0.250 mol dm3 sodium hydroxide at 298 K. [Kw = 1.001014 mol2dm6 at 298 K.] PMT (2) (1) (2) (2) (ii) Write the expression for the acid dissociation constant, Ka, for ethanoic acid. (iii) Calculate the pH of 0.125 mol dm3 ethanoic acid at 298 K. [Ka for ethanoic acid is 1.7105 mol dm3 at 298 K.] (iv) When half the ethanoic acid is neutralized, the concentration of the remaining ethanoic acid equals the concentration of the sodium ethanoate which has formed. What is the pH of the mixture at this point? Justify your answer. pHJustification18 *N36506A01824*<br />
PMT (v) On the axes below, sketch the titration curve for this reaction when 30 cm3 of the sodium hydroxide is added to 25.0 cm3 of the vinegar solution. (3) pH 14 12 10 10 15 20 25 30 Volume of NaOH added/cm3 *(vi) The only indicators which were available for this titration were methyl yellow (in ethanol) and thymolphthalein. Explain which indicator is more suitable for this titration and why the other is unsuitable. You will need to refer to your data booklet. (2)(b) In the food industry, ethanoic acid is described as an acidity regulator, additive number E260. Ethanoic acid can neutralize alkalis. What substance could be mixed with ethanoic acid so that it regulates pH as a buffer in foodstuffs? (1)(Total for Question 20 = 13 marks) TOTAL FOR SECTION B = 50 MARKS *N36506A01924* 19 Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/l_smalljpgJune_2010_QP_-_Unit_4_Edexcel_Chemistry_A-level_20-0.jpg)
![Question Number 20 (a)(i) Question Number 20 (a)(ii) Question Number 20 (a)(iii) Acceptable Answers Reject Mark Correct answer with or without working scores 2 marks [H+] = (1 .00 x 1014/ 0.250) = 4 x 1014 (1) pH = (13.39794 =) 13.4 (1) OR pOH = -log 0.250 = 0.602 (1) pH = (13.39794 =)13.4 (1) ALLOW TE in second mark if error in [H+] calculation gives pH more than 7 3 or more sf IGNORE rounding errors e.g. accept 13.39 Acceptable Answers Reject Mark [H+]2 [CH3COOH] (Ka =) [CH3COO][H+] (1) [CH3COOH] ALLOW H30+ instead of H+ [A][H+] if key to symbols given [HA] IGNORE state symbols Acceptable Answers Reject Mark Correct answer with or without working scores 2 marks 1.7 x 105 = [H+]2 (1) 0.125 [H+] = 1.46 x 103 pH = 2.84/2.8 (1) no TE from an incorrect [H+]<br />
Question Number 20 (a)(iv) Question Number 20 (a)(v) Acceptable Answers pH = 4.8 / 4.77 (1) pH = pKa / [H+] = Ka (when acid is half neutralized) (1) Reject H+ = Ka Mark Acceptable Answers Reject Mark Sigmoid curve starting between pH 2 and 4 (2.8), ending between pH 12 and 14 inclusive (1) with steep rise (may be vertical or gently sloping) of between 37 units between pH 6 and 12. Sloping section should not extend over more than 5cm3. (1) When 12.5 cm3, NaOH added. (1) ALLOW tolerance for grid Reverse curves lose first mark<br />
Question Number 20 (a)(vi) Acceptable Answers Reject Mark First mark Thymolphthalein more suitable as it changes (from colourless to blue) in steep region of titration (pH 8.3 to 10.6)/ at the equivalence point / at the end point OR thymolphthalein has pH range in steep region of titration (1) Second mark Methyl yellow changes (from red to yellow at pH 2.9 to 4) before equivalence point / before the end point / doesnt change in steep section OR Methyl yellow has pH range before / outside steep region of titration (1) ALLOW Thymolphthalein more suitable as it changes at the equivalence point but methyl yellow does not. This scores 2 marks OR First mark pKin1 must lie within vertical region on titration curve (1) Second mark hence thymolphthalein is suitable and methyl yellow is not (1) Question Number 20 (b) Acceptable Answers Reject Mark Sodium ethanoate/ CH3COONa Potassium ethanoate / CH3COOK ALLOW other cations as alternatives to sodium Use of sodium hydroxide (because its in food)<br />](https://learnah.org/wp-content/uploads/2018/08/June_2010_MS_-_Unit_4_Edexcel_Chemistry_A-level_20-0.jpg)
