Edexcel Jan 2017 (IAL) Paper 3 Q4 (with explained solutions) |

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Edexcel Jan 2017 (IAL) Paper 3 Q4

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4 A titration is carried out to determine the concentration of a solution of sodium hydroxide, NaOH(aq), using the organic acid H2Y. The equation for the reaction is H2Y(aq) + 2NaOH(aq)Na2Y(aq) + 2H2O(l) Sodium hydroxide solution is added from a burette to 25.0 cm3 of a 0.0500 mol dm3 solution of H2Y, to which several drops of phenolphthalein have been added. (a) State the colour change for the phenolphthalein indicator at the end-point of this titration. (2) Fromto(b) A student obtained the readings shown. Titration number Burette reading (final) / cm3 Burette reading (initial) / cm3 Volume of NaOH used / cm3 Used to calculate mean () 24.90 1.00 23.90 23.60 0.00 23.60 23.65 0.15 23.50 (i) Calculate the mean titre in cm3. Show which titres you have used in your calculation by putting a tick () in the appropriate boxes. (2) Mean titre =cm3 10 *P48371A01016*<br />
(ii) On the diagram below, show how the level of the sodium hydroxide solution appears when the final burette reading of 23.65 cm3 is recorded in titration 3. (2) 23 24 *P48371A01116* 11 Turn over<br />
(c) (i) Calculate the number of moles of the acid, H2Y, in 25.0 cm3 of a 0.0500 mol dm3 solution. (1) (ii) Calculate the number of moles of sodium hydroxide, NaOH, in the mean titre. (iii) Calculate the concentration of the sodium hydroxide solution in mol dm3. (iv) Describe two things you would do when using a burette to ensure that a particular reading is as accurate as possible. (1) (1) (2)(Total for Question 4 = 11 marks) 12 *P48371A01216*<br />

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Acceptable Answers Reject (From) colourless (1) (To) (pale) pink (1) ALLOW (1) for pink to colourless clear for colourless red / purple / magenta red-pink / purple-pink etc. Mark Acceptable Answers Reject One DP for final answer Ticks under titres 2 and 3 Check answer line first 23.55 (cm3) (1) (1) Allow correct mean for any combination of at least two ticked titres for the second mark Acceptable Answers Reject V shaped Mercury meniscus Straight line Shape of the meniscus correctly drawn. Allow any downward arc (1) Bottom of meniscus or top of upside down meniscus mid-way between 23.60 and 23.70 (1) Acceptable Answers Reject 0.05(00) x 25(.0) 1000 = 0.00125 / 1.25 x 103 (mol) IGNORE SF except 1 SF Mark Mark Mark Question Number 4(a) Question Number 4(b)(i) Question Number 4(b)(ii) Question Number 4(c)(i)<br />
Question Number 4(c)(ii) Question Number 4(c)(iii) Acceptable Answers Reject Answer to (c)(i) x 2 = 0.0025(0)/ 2.5(0) x 103 (mol) IGNORE SF except 1 SF Acceptable Answers Reject c(ii) x 1000 = 0.106 (mol dm3) 23.55 TE on mean titre IGNORE SF except 1 SF Mark Mark Question Number 4(c)(iv) Any TWO from:- Acceptable Answersuse white tile or paper as background to burette readingshave eyes level with meniscus (allow any angle that implies reading perpendicular to the scale or level with the meniscus)measure level at bottom of the meniscusensure that the burette is upright / vertical ALLOWensure that there are no air bubblesensure that the jet is fullremove the funnel from the top of the burette Mark Reject Just at eye level / perpendicular to burette Lower meniscus Comments regarding the use of the burette during the titration.use a white tile beneath flaskrinse with solution to be used Eyes level with the bottom of meniscus scores 2 (2) IGNORE Parallax error (Total for Question 4 = 11 marks)<br />

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