Edexcel Jan 2010 Paper 3 Q3 (with explained solutions) |

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Edexcel Jan 2010 Paper 3 Q3

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PMT 3 This question is about calcium hydroxide, Ca(OH)2. The solubility of calcium hydroxide in water can be found by titrating a saturated solution of calcium hydroxide with hydrochloric acid of known concentration. (a) Describe how you would make a saturated solution of calcium hydroxide suitable for use in this titration. Do not describe the subsequent titration procedure. (2)(b) 10.0 cm3 portions of the saturated solution are placed in a conical flask and titrated with 0.0500 mol dm3 hydrochloric acid added from a burette. (i) Name the apparatus used to measure the 10.0 cm3 portions. (1)(ii) Suggest a suitable indicator for this titration and state the colour change you would expect to see at the end-point. (2) IndicatorColour change fromto*N34104A0516* Turn over<br />
PMT (c) The following results were obtained: Titration numbers Final burette reading / cm3 Initial burette reading / cm3 Titre / cm3 19.20 10.00 9.20 28.05 19.20 8.85 37.10 28.15 (i) Fill in the third titre value in the table. (ii) Suggest why the first titre should be disregarded. (1) (1)(iii) Calculate the mean titre. (1) (iv) Calculate the number of moles of hydrochloric acid in the mean titre. (1)cm3 *N34104A0616*<br />
PMT (v) The equation for the reaction is Ca(OH)2(aq) + 2HCl(aq)CaCl2(aq) + 2H2O(l) Calculate the number of moles of calcium hydroxide in a 10.0cm3 portion of the saturated solution. (1) (vi) Calculate the concentration of calcium hydroxide in mol dm3. (vii) Calculate the solubility of calcium hydroxide in g dm3. (1) (1) *N34104A0716* Turn over<br />
PMT (d) The standard enthalpy change for the reaction of calcium hydroxide with hydrochloric acid was found by reacting 0.0100 mol of solid calcium hydroxide with 50.0 cm3 of a 1.00 mol dm3 solution of hydrochloric acid (an excess), in a polystyrene cup. The temperature rose from 21.2 C to 26.7 C. (i) Calculate the energy, in joules, transferred in the reaction. Use the expression Energy transferred = massspecific heat capacitytemperature change [Assume density of solution = 1.0 g cm3, specific heat capacity of solution = 4.18 J g1C 1] (ii) Calculate the standard enthalpy change, H , for the reaction. Include a sign and units in your answer. (iii) Calculate the percentage error in the temperature change caused by an uncertainty of 0.1C in each thermometer reading. (1) (2) (2) *N34104A0816*<br />
PMT (iv) The experiment was repeated using 50.0 cm3 of a 1.00 mol dm3 solution of nitric acid instead of the hydrochloric acid. Explain why the temperature change was the same in both experiments. (1)(v) The experiment was repeated again using 25 cm3 of 2.00 mol dm3 hydrochloric acid. Predict the temperature change in this experiment. (1)(vi) Which of the experiments in (iv) and (v) gave the least error in the temperature change? Justify your answer. (1)(Total for Question 3 = 20 marks) *N34104A0916* Turn over<br />

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Acceptable Answers Reject Add an excess of calcium hydroxide/calcium oxide to water /until no more will dissolve(1) OWTTE leave for 24 hours/heat and leave to cool/ decant/filter/keep temperature constant (1) Accept unexpected correct Chemistry! Add excess Ca to water Acceptable Answers Reject (10 cm3) pipette ALLOW recognisable spelling (eg pipet /pippette) Burette/measuring cylinder/cylinder Acceptable Answers Reject Litmus / universal indicator/ pH paper Indicator Methyl orange (1) From yellow to orange (1) ALLOW to red/pink ALLOW any named acid-base indicator (recognisable spelling) with correct colour change eg Phenolphthalein (1) From pink/purple/red to colourless (1) Acceptable Answers 8.95 (1) Acceptable Answers Reject Reject First titration is likely to be inaccurate/rough/only a range finder/ not concordant / not within0.2/0.1 cm3 Too high unless explained as overshoot Difference to high Acceptable Answers Reject 8.90 (1) ALLOW 8.9 ACCEPT TE from (i) Use of 9.20 (ans 9.00) Mark Mark Mark Mark Mark Mark Question Number 3 (a) Question Number 3 (b)(i) Question Number 3 (b)(ii) Question Number 3 (c)(i) Question Number 3 (c)(ii) Question Number 3 (c)(iii)<br />
Acceptable Answers 8.90 x 0.05 1000 = 4.45 x 10-4 / 4.45.10-4 / 0.000445 (mol) ALLOW Answer to (iii) x 0.00005 ALLOW 4.5x10-4/4.5.10-4/ 0.00045 Reject Mark Acceptable Answers Reject Mark 2.225 x 10-4 / 0.0002225 (mol) ALLOW Answer to (iv) divided by 2 ALLOW 2.23 x 10-4 / 0.000223 2.2 x 10-4 / 0.00022 2.3 x 10-4 / 0.00023 1 Mark Mark Acceptable Answers Reject 2.225 x 10-4 x 100 = 2.225 x 10-2 (mol dm- 3) ALLOW answer to (v) x 100 Reject Acceptable Answers 2.225 x 10-2 x 74.1 = 1.648725 = 1.65 (g dm-3) if Mr 74 the answer is 1.6465 = 1.65 (g dm-3) ALLOW Answer to (vi) x 74.1 or x 74 Ignore s.f. Note that 2.23 x 10-4 gives 1.650 with 74 or 1.652 with 74.1 2.3 gives 1.702 and 1.704 respectively 2.2 gives 1.628 and 1.630 respectively Question Number 3 (c)(iv) Question Number 3 (c)(v) Question Number 3 (c)(vi) Question Number 3 (c)(vii)<br />
Acceptable Answers Reject Mark 50.74 as mass Reject Mark Reject 1.81 % Reject Mark Mark Question Number 3 (d)(i) Question Number 3 (d)(ii) Question Number 3 (d)(iii) Question Number 3 (d)(iv) 50 x 4.18 x 5.5 = 1150/1149.5 (J) ALLOW 1.1/1.150/1.1495 kJ Acceptable Answers 1149.5 x10-3 0.01 =-114.95/-115 kJ mol-1 kJ/mol ALLOW -114950/-115000 J mol-1 Value (1) Sign and unit (1) ALLOW TE from (d)(i) ALLOW division by incorrect number of moles eg 1149.5 x10-3 (0) 0.05 = -22.99/-23.0 kJ mol-1 (1) internal TE for sign/unit but only for dividing by an incorrect number of moles. Acceptable Answers 0.2 x 100 5.5 = 3.6/3.64 % (2) 3.63 % (1max) First mark total error = 0.2 Second mark percentage ALLOW 0.1x100 = 1.8/1.82 % (1max) 5.5 Acceptable Answers as the same neutralisation reaction/ same ionic equation ALLOW as nitric acid is a strong acid /both acids the same strength /same volume of same concentration of acid /same numbers of moles of protons/H+ and same volume. Same numbers of moles of acid Question Number 3 (d)(v) Acceptable Answers 11 oC /doubles Reject increases Mark<br />
Reject Mark Mark Mark Mark Acceptable Answers Experiment (v)/ more concentrated hydrochloric acid A bigger temperature change (1) IF temperature rise is the same in (v) ALLOW TE eg (iv) as larger volume gives(lower percentage error in volume measurement) Also: A smaller temperature change in (v) means (iv) is more accurate Acceptable Answers 0.2 x 74.1 0.789 = 18.8 /18.78 (cm3) but check unit is not wrong ie dm3 ignore s.f. except one ALLOW 18.76/19 (from M = 74) Acceptable Answers Funnel with tap (1) ALLOW any shaped tube with opening at the top(which may be stoppered) with tap at the bottom Upper layer labelled as 2-chloro-2- methylpropane (1) Mark independently Reject 18.79 Reject Acceptable Answers Reject To remove/react with acid/neutralize (hydrochloric acid) Acceptable Answers Reject Mark No more bubbles/gas/pressure build up/carbon dioxide Use of indicators Acceptable Answers Reject (Anhydrous) sodium sulfate/calcium chloride/magnesium sulfate anhydrous copper sulfate /anhydrous cobalt chloride/concentrated sulfuric acid /calcium oxide Mark Question Number 3 (d)(vi) Question Number 4 (a) Question Number 4 (b) Question Number 4 (c)(i) Question Number 4 (c)(ii) Question Number 4 (d)<br />

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