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OCR A Jan 2013 Paper 5 Q7
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![This question looks at two weak acids that are used as food additives to preserve food: 16calcium hydrogensulfate(IV), Ca(HSO3)2 a carboxylic acid, HA. (a) Ca(HSO3)2 can be made by reacting an excess of sulfur dioxide gas with a suspension of calcium carbonate in water. Write the equation for this reaction.[1] (b) Calcium hydrogensulfate(IV), Ca(HSO3)2, dissolves in water forming an aqueous solution containing Ca2+(aq) and HSO3 (aq) ions. This solution is weakly acidic. (i) What is meant by a weak acid ? Write an equation to show why this solution is weakly acidic. weak acid:equation:[2] (ii) An aqueous solution of Ca(HSO3)2 oxidises magnesium forming hydrogen gas. Construct full and ionic equations for the oxidation of magnesium metal by Ca(HSO3)2(aq). full equation:ionic equation:[2] (iii) HSO3 (aq) can act as either a BrnstedLowry acid or a BrnstedLowry base. Explain this statement. Include equations for the reaction of HSO3 (aq) with H+(aq) and with OH(aq).[4]OCR 2013<br />
(c) A carboxylic acid HA is a food additive used as a preservative in cakes. 17 The Ka value of HA is 1.51105 mol dm3. A student analyses a sample of HA using the procedure below.A student dissolves 0.7369 g of HA in water and makes the solution up to 1.00 dm3. The student measures the pH of the resulting solution as 3.52. (i) Determine the molar mass of HA and suggest a possible formula for HA. HA has one carboxylic acid group and contains C, H and O only. Show all your working. molar mass of HA =g mol1 possible formula for HA =[6] (ii) The student had considered analysing the solution of HA by carrying out a titration with an alkaline solution of known concentration. The student rejects this method as being invalid because HA is a weak acid and only a small proportion of H+ ions would be neutralised. Explain whether the student was correct in rejecting the titration method.[1]OCR 2013 [Total: 16] Turn over<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN13_question_7-0.jpg)
![Mark Scheme January 2013 Marks 1 2 ALLOW multiples Guidance ALLOW ionisation for dissociationsign is required ALLOW multiples; state symbols not required DO NOT ALLOW equation with Ca2+ added to each side ALLOW multiples State symbols not required ALLOW as products: MgCa(SO3)2 + H2 DO NOT ALLOW Mg + Ca(HSO3)2 (cid:127)(cid:111) Mg2+ + Ca2+ + 2 SO3 ALLOW Mg + 2HSO3ASSUME It applied to HSO3 ALLOW equations withALLOW HSO3 Note: Final mark can only be awarded if both equations are correct(cid:127)(cid:111) Mg2+ + 2 SO3+ H+ (cid:127)(cid:111) H2SO3 2 + H2 2 + H2 F325 Question 7 (a) CaCO3 + 2SO2 + H2O (cid:127)(cid:111) Ca(HSO3)2 + CO2 (cid:57) Answer (b) (i) weak acid: partly dissociates (cid:57) HSO3H+ + SO3 2 (cid:57) (ii) Mg + Ca(HSO3)2 (cid:127)(cid:111) MgSO3 + CaSO3 + H2 (cid:57) Mg + 2H+ (cid:127)(cid:111) Mg2+ + H2 (cid:57) (iii) HSO3can accept a proton/H+ and donate a proton/H+ 2 (cid:57)+ OH (cid:127)(cid:111) H2O + SO3+ H+ (cid:127)(cid:111) H2O + SO2 (cid:57) OR Base accepts a proton/H+ AND Acid donates a proton/H+ (cid:57) HSO3 HSO3 Two correct equations linked to acid and base behaviour (cid:57) This could simply be labels (Acid AND base) for each equation, i.e. HSO3 HSO3+ OH (cid:127)(cid:111) H2O + SO3+ H+ (cid:127)(cid:111) H2O + SO2 Acid Base 2 18<br />
F325 Question (i) (c) Mark Scheme January 2013 Answer FIRST, CHECK THE ANSWER ON ANSWER LINE IF Mr = 122 award first 5 marks 6th mark is for formula -------------------------------------------------------------------- [H+] = 10pH = 103.52 = 3.02 x 104 (mol dm3) (cid:57) Marks [H+] [A(cid:16)] [HA] [H+]2 [HA] (cid:57) (3.02 (cid:117)10(cid:16)4)2 (cid:57) OR OR [HA] Ka = [HA] = (3.02 (cid:117)10(cid:16)4)2 1.51(cid:117)10(cid:16)5 [HA] = 6.04 x 103 (mol dm3) (cid:57) M = 0.7369 Carboxylic acid is C6H5COOH OR C7H6O2 (cid:57) 6.04 (cid:117)10(cid:16)3 = 122(.0) (g mol1) (cid:57) Guidance FULL ANNOTATIONS MUST BE USED --------------------------------------------------------------------------- Throughout calculation, ALLOW 3 significant figures up to calculator value correctly rounded ALLOW 3 SF to calculator value of 3.01995172 x 104 ALLOW any correct equation that shows the relationship between Ka, [H+], [A], [HA] Correct [HA] expression and calculation subsumes previous marks Using calculator [H+] value, [HA] = 6.039806883 x 103 Using calculator [HA] value, Mr = 122.0072122 ALLOW any feasible formula with a molar mass of 122 containing C, H AND at least two O atoms e.g. C6H2O3; C3H6O5 Note: a structural formula must contain COOH/CO2H ALLOW ECF for possible formula of HA from an incorrectly calculated molar mass of HA Note: the possible formula must be feasible and must contain C, H AND at least two O atoms IF [HA]eqm = [HA][H+] has been used, Mr = 116 and formula is C5H11COOH OR C6H12O2 ALL marks are available for this answer Calculator unrounded Mr = 116.1972565 19<br />
Mark Scheme January 2013 Guidance Statement AND reason required for the mark ALLOW incorrect AND equilibrium shifts to right Note: The key idea is that more H+ ions are produced by more dissociation A comment that all the H+ ions react is just repeating information in the question F325 Question (ii) Answer student is incorrect AND acid releases all H+ ions OR more acid dissociates (cid:57) Marks Total 16 20<br />](https://learnah.org/wp-content/uploads/2018/08/OCR_5_QP_JAN13_answer_7-0.jpg)
