Edexcel Jun 2011 Paper 3 Q4 (with explained solutions) |

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Edexcel Jun 2011 Paper 3 Q4

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PMT 4 An experiment was carried out to determine the concentration of a solution of potassium iodate(V), KIO3. Procedure 1. From a large volume of a solution of potassium iodate(V), use a pipette to withdraw a 25.00 cm3 sample and place the sample in a conical flask. 2. Add excess amounts of both potassium iodide solution and dilute sulfuric acid to the 25.00 cm3 of potassium iodate(V) solution in the conical flask. The iodate(V) ions oxidize iodide ions to iodine, in acidic solution, as shown in Equation 1: IO3+ 5I + 6H+ (cid:111) 3I2 + 3H2O (Equation 1) 3. Titrate the iodine formed with a solution of sodium thiosulfate, Na2S2O3, using starch as indicator. I2 + 2Na2S2O3 (cid:111) 2NaI + Na2S4O6 (Equation 2) Results Number of titration Burette reading (final) / cm3 Burette reading (initial) / cm3 Volume of Na2S2O3 used / cm3 25.10 0.00 25.10 26.35 2.05 24.30 24.10 0.00 24.10 (a) (i) Which two titrations should be used to calculate the mean (average) titre? Explain your answer. (2)*P37682A0816*<br />
(ii) Calculate the mean titre in cm3. (iii) The sodium thiosulfate solution used in the titration has a concentration of 0.100 mol dm3. Calculate the number of moles of sodium thiosulfate in the mean titre. (iv) Use Equation 2 to calculate the number of moles of iodine, I2, that reacted with the number of moles of sodium thiosulfate you have calculated in part (a)(iii). (v) Use your answer to part (a)(iv) and Equation 1 to calculate the number of moles of iodate(V) ions in 25.00 cm3 of solution. (vi) Use your answer to (a)(v) to calculate the concentration of the potassium iodate(V) solution, KIO3, in g dm3. [The molar mass of KIO3 is 214 g mol1] PMT (1) (1) (1) (1) (2) *P37682A0916* Turn over<br />
(b) (i) The error associated with reading the 25.00 cm3 volume of the potassium iodate(V) solution in a pipette is0.06 cm3. Calculate the percentage error associated with using a 25.00 cm3 pipette. (ii) Describe two things you could do to ensure that the burette readings are as accurate as possible. Assume that the burette has been appropriately rinsed and filled with the sodium thiosulfate solution. PMT (1) (2) 12(Total for Question 4 = 11 marks) 10 *P37682A01016*<br />
PMT BLANK PAGE *P37682A01116* 11 Turn over<br />

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Question Number 4 (a) (i) Acceptable Answers Reject Titres 2 and 3 (1) Concordant / agree within ()0.2 (cm3) / in close agreement/ titration 1 is a trial or rangefinder or a rough titration, etc. Question Number 4 (a) (ii) 24.20 / 24.2 (cm3) Acceptable Answers Mark cq on any titres chosen (1) Reject Acceptable Answers Reject Mark CQ on (a)(ii) 0.100 x 24.20 1000 = 0.00242 / 0.0024 / 2.42 x 103 / 2.4 x 103 (mol) Acceptable Answers Reject Question Number 4 (a) (iii) Question Number 4 (a) (iv) Answer to (a)(iii)2 = 0.00121 / 0.0012 / 1.21 x 103 / 1.2 x 103 (mol) Question Number 4 (a) (v) Answer to (a)(iv)3 Acceptable Answers = 0.000403 / 0.0004(0) / 4.03 x 104 / 4(.0) x 104 Reject Mark Mark Mark Mark Mark 6CH0701 1106<br />
Question Number 4 (a) (vi) Mark Acceptable Answers Reject EITHER Answer to (a)(v)0.025(0) = 0.0161 / 0.016(0) / 1.61 x 102 / 1.6(0) x 102 (mol dm3) (1) so concentration of KIO3 in g dm3 is 214 (g mol1) x 0.0161 (mol dm3) = 3.45 (g dm3) OR 214 (g mol1) x 0.016(0) (mol dm3) = 3.42 (g dm3) (1) OR Calculate mass KIO3 using 0.000403 x 214 = 0.086242 (g) (1) IGNORE units even if incorrect (then) 0.086242 (g) 0.025 (dm3) = 3.45 (g dm3) (1) ALLOW 2 s.f. but reject 1 s.f. NOTE: Check all calculations as final answer is dependent on accuracy in interim steps 6CH0701 1106<br />
Mark Mark Question Number 4 (b) (i) 0.06 x 100% = () 0.24 / Acceptable Answers Reject 0.2% 25.00 NOTE:not required for the mark Question Number 4 (b) (ii) Any TWO from:- Acceptable Answersuse white tile or paper as background to burette readingshave eyes level with meniscus (to avoid parallax error)measure level at bottom of the meniscusensure that the burette is upright ALLOWensure that there are no air bubblesensure that the jet is Reject Do more repeats / take more means Temperature of lab must be kept constant, etc full remove the funnel from the top of the buretteuse a white tile beneath flaskswirl the (conical) flaskrun solution from burette slowly (into conical flask)ensure no solution from the burette is left on the sides/walls of the (conical) flaskadd solution from burette drop-wise close to the end-point (2) 6CH0701 1106<br />

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