Cambridge A Level Chemistry 9701 — 2012 May/June Paper 5 · Variant 1

This document consists of 8 printed pages. DC (SJF/SW) 46939/4 © UCLES 2012 [Turn over UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS General Certificate of Education Advanced Level READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name on all the work you hand in. Write in dark blue or black pen. You may use a soft pencil for any diagrams, graphs or rough working. Do not use staples, paper clips, highlighters, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. You may lose marks if you do not show your working or if you do not use appropriate units. Use of a Data Booklet is unnecessary. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. * 9 9 5 3 6 0 8 1 2 2 * CHEMISTRY 9701/51 Paper 5 Planning, Analysis and Evaluation May/June 2012 1 hour 15 minutes Candidates answer on the Question Paper. No Additional Materials are required. For Examiner’s Use 1 2 Total

2 9701/51/M/J/12 © UCLES 2012 For Examiner’s Use 1 When an excess of aqueous sodium hydroxide, NaOH, is added to 100 cm3 of aqueous copper(II) sulfate, CuSO4, a precipitate of copper(II) hydroxide, Cu(OH)2, is produced. The stoichiometric equation for this reaction is, CuSO4 (aq) + 2NaOH (aq) Cu(OH)2 (s) + Na2SO4 (aq) The following information gives some of the hazards associated with these reactants. Copper(II) sulfate (solid hydrated copper(II) sulfate CuSO4.5H2O) Harmful. Dangerous for the environment. Harmful if swallowed. Irritating to eyes and skin. Solutions of concentrations equal to or greater than 1 mol dm–3 should be labelled HARMFUL. Sodium hydroxide (solid NaOH) Corrosive. Solutions of concentrations equal to or greater than 0.5 mol dm–3 are CORROSIVE. Solutions of concentrations equal to or greater than 0.05 mol dm–3 but less than 0.5 mol dm–3 are IRRITANT. You are to plan an experiment to investigate the molar ratio of the equation above and confirm that it remains unchanged as the concentration of the copper(II) sulfate changes. (a) (i) Predict quantitatively how the number of moles of the precipitated copper(II) hydroxide varies as the molar concentration of the copper(II) sulfate increases until saturation is reached. … … … (ii) Display your prediction in the form of a sketch graph. Remember that you are using 100 cm3 of aqueous copper(II) sulfate. Label clearly the point representing the saturated solution of copper(II) sulfate. A saturated solution at 25° C has a concentration of 1.39 mol dm–3. Give appropriate numerical scales to the two axes. 0 No of moles of copper(II) hydroxide concentration of the copper(II) sulfate / mol dm–3 0 [3]

3 9701/51/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use (b) In the experiment you are about to plan, identify the following. (i) the independent variable … (ii) the dependent variable … (iii) one other variable to be controlled …[2] (c) Design a laboratory experiment to investigate your prediction in (a). In addition to the standard apparatus present in a laboratory you are provided with the following materials. aqueous sodium hydroxide, NaOH (2.0 mol dm–3) solid hydrated copper(II) sulfate, CuSO4.5H2O Give a step-by-step description of how you would (i) prepare enough solutions of copper(II) sulfate of an appropriate range of concentrations to give sufficient data to plot a graph as in (a)(ii), (ii) collect and dry the precipitated copper(II) hydroxide, (iii) calculate the molar concentration of one of the solutions of copper(II) sulfate. [Ar: H, 1.0; O, 16.0; S, 32.1; Cu, 63.5] [5]

4 9701/51/M/J/12 © UCLES 2012 For Examiner’s Use (d) (i) State two hazards that must be considered when planning the experiment. … … … (ii) State a precaution that should be taken to minimise the risk of one of these hazards. … … …[3] (e) Draw up a table with appropriate headings to show the data you would record when carrying out your experiments and the values you would calculate in order to construct a graph to support or reject your prediction in (a). The headings should include the appropriate units. [2] [Total: 15]

5 9701/51/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use 2 The variation of the volume with temperature of a fixed mass of an ideal gas at constant pressure may be represented by a relationship known as Charles’s law, V = kT where V is the volume of a gas, T is the temperature in Kelvin and k is a constant. An experiment was carried out to attempt to verify this law. • A specially adapted gas syringe was filled with a sample of gas. The syringe was placed in a temperature controlled chamber at 25 °C and left for 5 minutes. The initial volume of gas at this temperature was 26.0 cm3. • The temperature was adjusted and, after leaving for 5 minutes, the change in gas volume for the new temperature was recorded. • The experiment was repeated several times at different temperatures and the results recorded. (a) The results of the experiment are recorded below. Process the results in the table to calculate the volume of the gas and the corresponding temperature in Kelvin to enable you to plot a graph to show their inter-relationship. (Note 0 °C is 273 K). Record these values to three significant figures in the additional columns of the table. You may use some or all of the columns. Label the columns you use. For each column you use include units where appropriate and an expression to show how your values are calculated. You may use the column headings A to E in these expressions (e.g. A–B). A B C D E temperature of the gas /°C change in volume of the gas /cm3 –23 –4.20 –4 –2.50 11 –2.10 29 0.40 42 1.50 63 3.30 77 4.50 94 7.00 117 8.00 131 9.20 [2]

6 9701/51/M/J/12 © UCLES 2012 (b) Present the data calculated in (a) in graphical form. Draw the line of best fit. [3]

7 9701/51/M/J/12 © UCLES 2012 [Turn over For Examiner’s Use (c) Circle and label on the graph any point(s) you consider to be anomalous. For each anomalous point give a different reason why it is anomalous, clearly indicating which point(s) you are describing. … … … … … …[3] (d) (i) Determine the slope of the graph. Mark clearly on the graph any construction lines and show clearly in your calculation how the intercepts were used in the calculation of the slope. (ii) Record the volume of the gas at 273 K. [3] (e) Comment on the reliability of the data provided in (a). [1]

8 9701/51/M/J/12 © UCLES 2012 For Examiner’s Use (f) Indicate whether the results do or do not confirm Charles’s law. … Give a reason for your answer. … … … …[2] (g) As the pressure of a gas is reduced its volume increases. On your graph on page 6 draw another line showing how the volume varies with temperature if the experiment were repeated using the same fixed mass of gas at a lower pressure. [1] [Total: 15] Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. University of Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge.

UNIVERSITY OF CAMBRIDGE INTERNATIONAL EXAMINATIONS GCE Advanced Subsidiary Level and GCE Advanced Level MARK SCHEME for the May/June 2012 question paper for the guidance of teachers 9701 CHEMISTRY 9701/51 Paper 5 (Planning, Analysis and Evaluation), maximum raw mark 30 This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes must be read in conjunction with the question papers and the report on the examination. • Cambridge will not enter into discussions or correspondence in connection with these mark schemes. Cambridge is publishing the mark schemes for the May/June 2012 question papers for most IGCSE, GCE Advanced Level and Advanced Subsidiary Level syllabuses and some Ordinary Level syllabuses.

Page 2 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 51 © University of Cambridge International Examinations 2012 Question Sections Indicative material Mark 1 (a) (i) (ii) PLAN Problem States that the moles of copper(II) hydroxide increase as the molar concentration of copper(II) sulfate increases and sketches a line from the origin with an initial positive gradient. Ignore any subsequent plateau or maximum on this line. A straight line terminating at the point of saturation with marked co-ordinates: award 2 marks. A straight line clearly terminating within the grid but without marked co-ordinates: award 1 mark. A line (not necessarily straight) which does not terminate at the saturation point but with the co-ordinates marked: award 1 mark. This line can plateau after the saturation point. [1] [1] [1] (b) PLAN Problem (i) concentration of copper(II) sulfate. (ii) moles of copper(II) hydroxide [1] [1] (c) PLAN Method Indicates at least five experiments. These may be shown in the table in 1(e). Five blank rows in the table are acceptable. A range of concentrations over at least 0.8 mol dm–3, which must cover 1.0 mol dm–3, up to a maximum of 1.39. Accept a range of mass of copper(II) sulfate (with solution volume) that has been calculated satisfying the same concentration criteria. Filtering/centrifuging Method of drying and weighing the precipitate. Include washing with water (and propanone), (air) drying and weighing (to constant mass). Do not accept direct heating, blotting or a statement that the precipitate is simply left to dry. A suitable calculation of a molarity, even if greater than 1.39 (Mr of the copper(II) sulfate must be used). Check that the solution is made up to the appropriate volume and not that a mass is added to a fixed volume of water. [1] [1] [1] [1] [1] (d) PLAN Method Identifies that copper(II) sulfate is harmful/a danger to the environment. Identifies that sodium hydroxide is corrosive (from the hazcard information). Give one mark for a precaution for either hazard of (chemical) resistant gloves or large dilution when disposing of chemicals. [1] [1] [1]

Page 3 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 51 © University of Cambridge International Examinations 2012 (e) PLAN Method This table must match the plan in 1(c). Five or four columns are required depending on whether serial dilution is used to prepare the solutions: Either (i) mass of copper(II) sulfate/g or (ii) volume of copper sulfate solution/cm3 and volume of water/cm3 for serial dilutions; mass/weight of copper(II) hydroxide/g; concentration of copper(II) sulfate/mol.dm–3; number of moles of copper(II) hydroxide (no unit). The full word for the unit can be used with or without / or (). Ignore other column headings and units. If Five or Four are fully correct, two marks; four or three correct, one mark; otherwise zero. [2] [Total: 15]

Page 4 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 51 © University of Cambridge International Examinations 2012 2 (a) ACE Data The required two column headings temperature, (A + 273), /K and volume, (B + 26), /cm3 are fully correct. The full word for the unit can be used with or without / or (). Both columns are fully completed to the correct number of significant figures. Allow 2 errors. [1] [1] (b) ACE Data Label the x-axis temperature and the y-axis volume. Any correct unambiguous label is acceptable e.g. column label (A). Units must be present in the accepted forms. The plotted points must cover at least half the grid in both directions and all points must be on the given grid. If a true origin has been scaled in and the candidate’s line is extrapolated back to there then the origin is to be included as a “plotted point” in the assessment of covering half the grid in both directions. This mark not available for other plots. Check the plotting of points 1, 3, 8, 10 and any obvious error. Give one mark for drawing a ‘straight-line of best fit’ (allow ‘ecf’ here from incorrectly plotted points). [1] [1]* [1]* (c) ACE Evaluation Incorrect plots of (i) volume/temp °C and (ii) volume change/temp will still allow these marks to be accessed. Any other wrong plots will not. Allow the candidate to select up to five anomalies which must include that furthest from the line. All the anomalous points are circled on the grid or unambiguously stated in the text. For each of the two different anomalies an appropriate explanation gains one mark. Point 3 (V low). Gas not equilibrated with the increased temperature, OR volume read before all the gas attained the increased temperature. OR volume read before 5 minutes at the increased temperature. Point 8 (V high). Gas not equilibrated with the decreased temperature, OR volume read before all the gas attained the decreased temperature. OR volume read before 5 minutes at the decreased temperature. If the candidate suggests that the gas is not equilibriated for both anomalous points but does not specify the direction of temperature change to that point, award 1 mark. [1] [2]

Page 5 Mark Scheme: Teachers’ version Syllabus Paper GCE AS/A LEVEL – May/June 2012 9701 51 © University of Cambridge International Examinations 2012 (d) ACE data (i) For, two pairs of construction lines on the graph drawn from the line to the axes and for correctly deducing the graph values of these two intercepts, give one mark. There has to be some indication on the graph of the intercepts used to calculate the gradient. These could be table points, provided they are indicated on the graph. If the true origin has been used in calculating the slope then only one pair of lines and one intercept is necessary. A correctly calculated value of the slope using the candidate’s figures. The mark is for the magnitude (ignore units). If the candidate used the true origin in the slope calculation then two zeros are not needed in the calculation. If the slope expression is inverted, then the calculation mark is lost but the intercept value mark can be gained. (ii) For a correctly read intercept at 273K give one mark. This mark is available for other permitted wrong plots as in 2(c), but for a volume change/temp plot 26 cm3 has to be added to the read intercept. Allow a calculated volume only if the candidates line on the graph was extrapolated to the true origin. Units must be included. [1]* [1]* [1] (e) ACE Evaluation The data is reliable as most (6 or more) of the points/results/data lie on the line of best fit. Accept few anomalous points. If there are 6 or more points not on the line then accept that the results are not reliable. [1] (f) ACE Conclusion These marks not available for other plots. For a statement that the ‘law’ is justified because a straight line (direct proportionality) is obtained give one mark. The data confirms the relationship V = kT or that volume is directly proportional to absolute temperature. Calculations showing that V/T is the same for more than 1 point on the graph are worth 1 mark. [1] [1] (g) ACE Conclusion A second line on the grid above the original line. This second line to have a greater slope (not parallel) and not touching the original line unless at the true origin (if used). Again, this mark is available for the other permitted plots as in 2(c). [1] (* is mark available for other plots) [Total: 15]