OCR A Jun 2013 Paper 1 Q2 (with explained solutions) |

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OCR A Jun 2013 Paper 1 Q2

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2 Hydrated aluminium sulfate, Al2(SO4)3x H2O, and chlorine, Cl2, are used in water treatment. (a) A student attempts to prepare hydrated aluminium sulfate by the following method. The student heats dilute sulfuric acid with an excess of solid aluminium oxide. The student filters off the excess aluminium oxide to obtain a colourless solution of Al2(SO4)3. (i) State the formulae of the two main ions present in the solution of Al2(SO4)3.and[2] (ii) Write an equation for the reaction of aluminium oxide, Al2O3, with sulfuric acid. Include state symbols.[2] (iii) What does x H2O represent in the formula Al2(SO4)3x H2O?[1] (iv) The student heats 12.606 g of Al2(SO4)3x H2O crystals to constant mass. The anhydrous aluminium sulfate formed has a mass of 6.846 g. Use the students results to calculate the value of x. The molar mass of Al2(SO4)3 = 342.3 g mol1. x =[3]OCR 2013<br />
(b) A student tests chlorine gas with damp blue litmus paper. The litmus paper first turns a red colour and is then bleached. A reaction takes place between chlorine and water in the damp litmus paper. (i) Write the equation for the reaction between chlorine and water. Explain why the damp litmus paper turns a red colour as a result of this reaction.[2] (ii) Bleach is made by reacting chlorine with cold dilute aqueous sodium hydroxide. Suggest the formula of the ion responsible for bleaching.[1] [Total: 11]OCR 2013 Turn over<br />

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Question 2 (a) (i) Al3+ (cid:57) SO4 2 (cid:57) F321 Answer Mark Scheme Marks (ii) Al2O3(s) + 3H2SO4(aq) (cid:198) Al2(SO4)3(aq) + 3H2O(l) Correct species AND correctly balanced (cid:57) state symbols on correct species (cid:57) (The number of) water(s) of crystallisation (cid:57) (iii) (iv) First check the answer on the answer line. If answer = 16, award 3 marks Correctly calculates amount of Al2(SO4)3: 6.846 / 342.3 = 0.02(00) mol (cid:57) Correctly calculates amount of H2O: 5.760 / 18.0 = 0.32(0) mol (cid:57) Correctly calculates whole number ratio of mol of H2O: Al2(SO4)3 to give x = 16 (cid:57) June 2013 Guidance ALLOW multiples IGNORE hydrated OR hydrous OR contains water If there is an alternative answer, check to see if there is any ECF credit possible using working below ALLOW as ECF from 12.606/342.3 = 0.0368(273) AND 0.32/0.0368(273) To give x = 9 for two marks ALLOW calculator value or rounding to 2 significant figures or more BUT IGNORE trailing zeroes, eg 0.200 allowed as 0.2. ALLOW ECF for calculation of correctly rounded whole number value of H2O from incorrect mol of H2O and / or incorrect mol of Al2(SO4)3 BUT x must be a whole number ALLOW alternative method Mol of Al2(SO4)3: 6.846 / 342.3 = 0.02(00) mol (first mark) Molar mass of Al2(SO4)3(cid:120)xH2O: 12.606 / 0.02(00) = 630.3 g mol 1 (second mark) Mass of water per mol = 630.3342.3 = 288 AND 288/18 to give x = 16 (third mark)<br />
F321 Mark Scheme June 2013 Question 2 (b) Answer (i) Cl2 + H2O (cid:198) HCl + HClO (cid:57) H+ ions are released OR HCl is acidic OR HClO is acidic (cid:57) Marks Guidance ALLOW HOCl ALLOW equilibrium sign IGNORE state symbols ALLOW formulae OR names If correct equation is seen: ALLOW product is acidic OR acid is produced IGNORE the solution is acidic but ALLOW the solution formed is acidic DO NOT ALLOW chlorine is acidic ie acidity must be related to the product(s) If an incorrect equation is seen: ALLOW second mark if H+ OR HCl OR HClO is given as a product in the equation AND is stated as being acidic If no equation is seen: ALLOW second mark if H+ OR HCl OR HClO is produced AND is stated as being acidic ALLOW OCl (ii) ClO (cid:57) Total 11<br />
F321 Question (i) 3 (a) Mark Scheme June 2013 Marks Answer P in P4 is 0 AND in PH3 is 3 AND in NaH2PO2 is (+)1 (cid:57) Phosphorus has been oxidised (from 0) to +1(cid:57) Phosphorus has been reduced (from 0) to 3 (cid:57) Guidance FULL ANNOTATIONS WITH TICKS, CROSSES, CON, etc MUST BE USED ALLOW oxidation states written above the equation if not seen in the text BUT IGNORE oxidation states written above the equation if seen in the text ALLOW 3 AND 1+ DO NOT ALLOW ions DO NOT ALLOW P3 in PH3 OR P+ in NaH2PO2 DO NOT ALLOW phosphide or phosphine or phosphate in place of phosphorus ALLOW P or P4 for phosphorus ALLOW ECF for the second and third marks if ONE incorrect oxidation number is assigned but directional changes are correct eg P = 0 and 3 and +2 instead of 0 and 3 and +1. IGNORE references to electron loss / gain If correct oxidation numbers are seen ALLOW second AND third marking points for: Phosphorus is oxidised to form NaH2PO2 AND Phosphorus is reduced to form PH3 IF neither second and third marks have been awarded ALLOW for ONE mark: Phosphorus has been both oxidised and reduced OR Phosphoruss oxidation number has increased and decreased<br />

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