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CIE Jun 2014 v1 Paper 3 Q1
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![Hydrogen peroxide, H2O2, is unstable and decomposes to give water and oxygen. In addition to the usual units of concentration, mol dm3, the concentration of a solution of hydrogen peroxide can also be given in volume strength or vol. For example, in Question 3 you will use a solution of 20 vol H2O2. This term means that when 1 dm3 of this solution is completely decomposed it generates 20 dm3 of oxygen at room temperature and pressure. PMT The aim of the following titration is to determine the volume strength of a solution of hydrogen peroxide. To do this you will titrate an acidi ed solution of hydrogen peroxide with potassium manganate(VII) solution. FA 1 is 0.0200 mol dm3 potassium manganate(VII), KMnO4. FA 2 is aqueous hydrogen peroxide, H2O2. FA 3 is 1.0 mol dm3 sulfuric acid, H2SO4. (a) MethodFill the burette with FA 1. Pipette 25.0 cm3 of FA 2 into the conical flask. Use the measuring cylinder to add 25 cm3 of FA 3 to the conical flask. Run FA 1 from the burette into the conical flask until the pink colour remains. Perform a rough titration and record your burette readings in the space below. The rough titre iscm3. Carry out as many accurate titrations as you think necessary to obtain consistent results. Make certain any recorded results show the precision of your practical work. Record, in a suitable form below, all of your burette readings and the volume of FA 1 added in each accurate titration. II III IV VI [6]UCLES 2014 9701/31/M/J/14<br />
(b) From your accurate titration results obtain a suitable value to be used in your calculations. Show clearly how you obtained this value. PMT 25.0 cm3 of FA 2 requiredcm3 of FA 1. [1] (c) Calculations Show your working and appropriate signi cantgures in thenal answer to each step of your calculations. (i) Calculate the number of moles of potassium manganate(VII) present in the volume of FA 1 calculated in (b). moles of KMnO4 =mol (ii) Use the following equation and your answer to (i) to calculate the number of moles of hydrogen peroxide used in each titration. 2KMnO4 + 5H2O2 + 3H2SO4K2SO4 + 2MnSO4 + 5O2 + 8H2O (iii) Calculate the concentration, in mol dm3, of H2O2 in FA 2. moles of H2O2 =mol concentration of H2O2 =mol dm3 (iv) Write an equation for the decomposition of hydrogen peroxide to produce oxygen and water.(v) Calculate the concentration in volume strength of H2O2 in FA 2. You must assume that the molar volume of oxygen is 24.0 dm3 mol1 at room temperature and pressure. II III IVUCLES 2014 9701/31/M/J/14 concentration of FA 2 =vol [5] [Total: 12] [Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_v1_QP_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
![Page 2 Mark Scheme GCE AS/A LEVELMay/June 2014 Syllabus 9701 Paper 31 Question Sections Indicative material Mark Total (a) PDO Layout I Initial and final readings and titre value given for rough titre and initial and final readings for two (or more) accurate titrations (minimum of 22 box) PDO Recording initial/start (burette) reading/volume final/end (burette) reading/volume titre or volume/FA 1 used/added (not difference) II Appropriate headings and units for all accurate data. and volume FA 1 added recorded for each accurate titre. Headings should match readings.unit: /cm3 or (cm3) or in cm3 or cm3 for each entry III All accurate burette readings recorded to 0.05 cm3. The need to record to 0.05 applies only to the burette readings and not to the recorded titres. Do not award this mark if:more than one final burette reading is 50.(00)50(.00) is used as an initial burette reading any burette reading is greater than 50.(00). MMO Decisions IV Has two uncorrected, accurate titres within 0.1 cm3 Do not include a reading labelled rough. Do not award this mark if, having performed two titres within 0.1 cm3, a further titration is performed that is more than 0.1 cm3 from the closer of the two initial titres unless further titrations within 0.1 cm3 of any other have also been carried out. Do not award the mark if any accurate burette readings (apart from initial 0) are given to zero dp. Examiner rounds burette readings to the nearest 0.05 cm3, checks subtractions and then selects the best titres using the hierarchy: two (or more) identical, then two (or more) within 0.05 cm3, then two (or more) within 0.1 cm3, etc. Examiner compares candidate mean titre with Supervisor mean titre. (a) MMO Quality Award V and VI for difference from Supervisor,0.20 cm3 Award V only for 0.20 <0.40 cm3 Spread penalty: if the two best titres are0.50 cm3 apart cancel one of the Q marks. [6]Cambridge International Examinations 2014<br />
Page 3 Mark Scheme GCE AS/A LEVELMay/June 2014 Syllabus 9701 Paper 31 Question Sections Indicative material Mark Total (b) ACE Interpretation Candidate must average two (or more) titres that are within 0.20 cm3. Working must be shown or ticks must be put next to the two (or more) accurate readings selected. The mean should normally be quoted to 2 dp rounded to the nearest 0.01. Two special cases where the mean may not be to 2 dp: allow mean to 3 dp only for 0.025 or 0.075 e.g. 26.325; allow mean to 1 dp if all accurate burette readings were given to 1 dp and the mean is exactly correct. e.g. 26.0 and 26.2 = 26.1 is correct but 26.0 and 26.1 = 26.1 is incorrect. Note: the candidates mean will sometimes be marked as correct even if it is different from the mean calculated by the Examiner for the purpose of assessing accuracy. (c) ACE Interpretation I Correctly evaluates 0.0200 (b) 1000 in (i) PDO Display II Correctly evaluates 5/2(i) 25 in (iii) III Correct balanced equation in (iv) ACE Interpretation IV Correctly evaluates ans (iii)24.0 in (v) (Allow ecf from incorrect equation) PDO Display V All answers given to 3 or 4 sf (minimum of 3 answers attempted) Qn 1 Total [1] [5] [12]Cambridge International Examinations 2014<br />](https://learnah.org/wp-content/uploads/2018/08/June_2014_v1_MS_-_Paper_3_CIE_Chemistry_A-level_1-0.jpg)
