Edexcel Jun 2015 (IAL) Paper 2 Q21 (with explained solutions) |

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Edexcel Jun 2015 (IAL) Paper 2 Q21

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 21 This is a question about an acid-base titration. Potassium hydroxide, KOH, is used to assist in the removal of hair. For example, it is present in some pre-shave products and used in solutions for soaking animal skins prior to the removal of the animal hair. The skin of a red-brown cow was soaked in a solution of potassium hydroxide containing 226.8 g of potassium hydroxide in 45.0 dm3 of solution. After several hours, the skin was removed. The residual solution, R, contained unreacted potassium hydroxide. In order to determine the potassium hydroxide concentration in R, 25.00 cm3 samples of the solution were titrated with 0.0500 mol dm3 sulfuric acid. Titration Final volume / cm3 Initial volume / cm3 Volume added / cm3 Trial 5.00 0.00 5.00 9.50 5.00 4.50 14.10 9.55 4.55 18.55 14.10 4.45 Mean titre = 4.50 cm3 The equation for the reaction is: 2KOH(aq) + H2SO4(aq) (cid:111) K2SO4(aq) + 2H2O(l) (a) (i) Calculate the number of moles of sulfuric acid that react with 25.00 cm3 of the potassium hydroxide solution R. (ii) From your answer to (a)(i), deduce the number of moles of potassium hydroxide in the 25.00 cm3 of solution R. 10 *P44882A01024* (1) (1)<br />
(iii) Calculate the concentration, in mol dm3, of potassium hydroxide in the solution R. (iv) Calculate the difference between the initial concentration of the potassium hydroxide used to soak the animal skin and the concentration of solution R, which you have calculated in (a)(iii). Relative Atomic Masses: K = 39.1; O = 16; H = 1 (1) (3) Initial KOH ConcentrationKOH concentration in solution RDifference(v) Calculate the total mass of potassium hydroxide used up in the soaking process. Give your answer to three significant figures. (2) *P44882A01124* 11 Turn over<br />
(b) The indicator phenolphthalein could have been used for this titration. (i) State the colour change you would expect at the end-point of a titration when sulfuric acid is added to potassium hydroxide using phenolphthalein. (2) Fromto(ii) Suggest why the particular skin used might make it difficult to accurately judge the end-point of the titration. (1)(iii) Phenolphthalein is used as a solution in ethanol which is highly flammable. A student suggested that for safety reasons there should be no naked flames present during this titration. Is this an appropriate suggestion? Justify your answer. (1)(c) Titration experiments use equipment with a measurement uncertainty. For a pipette, the uncertainty is 0.06 cm3 on the volume measured. For each burette reading, the uncertainty is 0.05 cm3. (i) By calculating the percentage error for the burette titre value of 4.50 cm3, and for the pipette volume of 25.00 cm3, show that in this case the burette error is greater than the pipette error. (2) Burette titre % errorPipette volume % error12 *P44882A01224*<br />
(ii) Suggest two ways by which the percentage error for the burette titre could be reduced, without changing the apparatus. (2) 12(iii) The trial titre value was not included in the calculation of the mean. In what circumstances could the trial value be used in the calculation of the mean? (1)(Total for Question 21 = 17 marks) *P44882A01324* 13 Turn over<br />

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Reject Reject Mark Mark Acceptable Answers Question Number 21(a)(iii) (c=4.50 x 10-40.025=) Reject Mark 1.8 x 10-2 / 0.018 / 1.80 x 10-2 /0.0180 (mol dm-3) TE ans to (a)(ii) 0.025 IGNORE SF except 1SF Section B Question Number 21(a)(i) Acceptable Answers (n=0.05 x 0.00450=) 2.25 x 10-4 / 0.000225 (mol) IGNORE SF except 1SF Question Number 21(a)(ii) (n=2.25 x 10-4 x 2=) Acceptable Answers 4.50 x 10-4 / 0.000450 (mol) TE ans to (a)(i) x 2 IGNORE SF except 1SF<br />
Acceptable Answers Reject Mark 0.07 Start at final answer (the difference) if correct or correct TE from (iii) then give 3 marks with or without correct working Ignore SF except 1SF for the difference only but do not penalise trailing zeros NOTE Negative value for difference does not get MP3 but can score MP1 and MP2 only If answer is incorrect then look at following working MP1 Initial KOH concentration n=226.856.1= (1) 4.04278/4.04 (mol) ALLOW use of 56 MP2 [KOH]= 4.0427845 = (1) 8.9840 x 10-2 / 0.089840 (mol dm-3 ) NOTE 56 gives 0.09 A TE is allowed from incorrect number of mols MP3 Difference (8.9840 x 10-21.80 x 10-2 =) 7.1840 x 10-2 / 0.071840 (mol dm-3) NOTE 56 gives 0.072 (1) Transferred errors 8.98 x 10-2ans to (a)(iii) OR Their initial concentration of KOHans to (a)(iii) COMMENT A difference of 0.071 means there has been a rounding error and so will score 2 marks only if rounding errors have not already been penalised. Question Number 21(a)(iv)<br />
Acceptable Answers Question Number 21(a)(v) Correct final answer (181/182) to 3SF with or without working scores (2) Answer to (iv) x 45 OR x 56.1 (1) n=7.18 x 10-2x 45= (3.231) (mol) m=3.231 x 56.1 = (181.359 / 181.4) OR 181.2591 / 181.3 = 181 (g) NOTE ALLOW USE OF 56 (1) Alternative method Answer to (ii) x 45000 OR x 56/56.1 (1) 25 Amount = 0.81 (mol) Mass of KOH left 0.81 x 56.1/56 = 45.441/45.36 (g) Mass used = 226.845.441/45.36 = 181 (g) (1) Reject Mark Mark 0 1 1 2 Some TE values: Part (i) Answer 25 x 0.05 1000 = 1.25 x 10-3 2.5 x 10-3 0.1 0.0890.1 = -0.0102 0.459 (mol) and 25.7 (g) Mark Answer 0 25 x 4.5 1000 = 0.1125 0.225 9 0.089 - 9 = -8.91 400.95 (mol) and 22 500 (g) 1 1 2 (ii) (iii) (iv) (v)<br />
Reject purple clear Reject Mark Mark Reject Mark Yes Acceptable Answers Question Number 21(b)(i) (From) (pale/bright) pink/red (1) (To) colourless (1) ALLOW one mark for colourless to pink/red (1) Second mark dependant on shade of pink/red/purple for first colour Question Number 21(b)(ii) Red/brown/colour (from the hair/skin Acceptable Answers likely to have) leached out/dissolved/ solution formed ALLOW Red/brown/colour from the hair/skin makes the (colour) change/end point difficult to judge/see Acceptable Answers Question Number 21(b)(iii) No Only a few drops of indicator used OR Adding to an aqueous solution OR Ethanol mixes with water (in all proportions) ALLOW Ethanol is in solution IGNORE Any other reasons<br />
Acceptable Answers Question Number 21(c)(i) (Titre error) (0.05 x 2 x 100=) 2.2(2) (%) (1) 4.50 (Sample error) (0.06 x 100=) 0.24 (%) (1) 25 Question Number 21(c)(ii) Mark each point independently Acceptable Answers Reject Mark Reject Mark Use more skin Any two from: Reduce the concentration of the sulfuric acid (1) Use a larger (initial) sample/R/KOH volume (1) Use HCl(aq) (of same concentration as sulfuric acid which would have a larger titre) (1) Use greater (initial) concentration/mass of KOH (1) Use less skin (1) IGNORE (Just) use larger titre Repeat the titration Just changing the concentration<br />
Acceptable Answers Question Number 21(c)(iii) When it is concordant/the same Reject Mark OR Within 0.1 (cm3) of the (mean of) other titres ALLOW Within 0.2 (cm3) of the other titres (comment this is as per the User guide) IGNORE Close/similar/almost the same as other titres TOTAL FOR QUESTION 21 = 17 MARKS<br />

Question 18 19 20 21 22 23

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