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AQA Specimen Paper 3 Q4
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![10 Ethanedioic acid is a weak acid. Ethanedioic acid acts, initially, as a monoprotic acid. [6 marks] Use the concept of electronegativity to justify why the acid strengths of ethanedioic acid and ethanoic acid are different. Typesetter code 0 4 . 1 Barcode<br />
. 2 A buffer solution is made by adding 6.00102 mol of sodium hydroxide to a solution containing 1.00101 mol of ethanedioic acid (H2C2O4). Assume that the sodium hydroxide reacts as shown in the following equation and that in this buffer solution, the ethanedioic acid behaves as a monoprotic acid. 11 H2C2O4(aq) + OH(aq) HC2O4 (aq) + H2O(l) The dissociation constant Ka for ethanedioic acid is 5.89102 mol dm3. [5 marks] Calculate a value for the pH of the buffer solution. Give your answer to the appropriate number of significant figures. pH = Question 4 continues on the next page 0 4 Barcode Typesetter code Turn over <br />
12 . 3 0 4 In a titration, the end point was reached when 25.0 cm3 of an acidified solution containing ethanedioic acid reacted with 20.20 cm3 of 2.00 102 mol dm3 potassium manganate(VII) solution. Deduce an equation for the reaction that occurs and use it to calculate the original concentration of the ethanedioic acid solution. [4 marks] Equation Calculation Original concentration = mol dm3 Barcode Typesetter code<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_QP_-_Paper_3_AQA_Chemistry_A-Level_4-0.jpg)
![Question 04.1 Marking guidance This question is marked using levels of response. Refer to the Mark Scheme Instructions for Examiners for guidance on how to mark this question. Level 3 56 marks All stages are covered and the explanation of each stage is generally correct and virtually complete. Answer is communicated coherently and shows a logical progression from stage 1 and stage 2 to stage 3. Steps in stage 3 must be complete, ordered and include a comparison. All stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies OR two stages are covered and the explanations are generally correct and virtually complete. Answer is mainly coherent and shows aprogression from Stage 1 and stage 2 to stage 3. Two stages are covered but the explanation of each stage may be incomplete or may contain inaccuracies, OR only one stage is covered but the explanation is generally correct and virtually complete. Answer includes some isolated statements, but these are not presented in a logical order or show confused reasoning. Insufficient correct Chemistry to warrant a mark. Level 2 34 marks Level 1 12 marks Level 0 0 marks Mark AO AO3 1a MARK SCHEMEA-level ChemistrySpecimen paper 3 Comments Indicative Chemistry content Stage 1: difference in structure of the two acidsThe acids are of the form RCOOHbut in ethanoic acid R = CH3whilst in ethanedioic acid R = COOH Stage 2: the inductive effectThe unionised COOH group contains two very electronegative oxygen atomstherefore has a negative inductive (electron withdrawing) effectThe CH3 group has a positive inductive (electron pushing) effect Stage 3: how the polarity of OH affects acid strengthThe OH bond in the ethanedioic acid is more polarised / H becomes more +More dissociation into H+ ionsEthanedioic acid is stronger than ethanoic acid 7 of 10<br />
Moles of NaOH = Moles of HOOCCOO formed = 6.00102 Moles of HOOCCOOH remaining = 1.001016.00102 = 4.00102 Ka = [H+][A]/[HA] [H+] = Ka[HA]/[A] [H+] = 5.89102(4.00102/V)/(6.00102/V) = 3.927102 pH = log10(3.927 102) = 1.406 = 1.41 2 + 16H+ + 2MnO42Mn2+ + 10CO2 + 8H2O2Mn2+ + 10CO2 + 8H2O 5H2C2O4 + 6H+ + 2MnO4 OR 5C2O4 Moles of KMnO4 = 20.22.00102/1000 = 4.04104 Moles of H2C2O4 = 5/24.04104 = 1.01103 Concentration = moles/volume (in dm3) = 1.011031000/25 = 4.04102 (mol dm3) 04.2 04.3 MARK SCHEMEA-level ChemistrySpecimen paper 3 1 1 1 1 1 1 1 AO2h AO2h AO2h AO2h AO1b AO2d AO2h AO2h AO2h Extended response Answer must be given to this precision If 1:1 ratio or incorrect ratio used, M2 and M4 can be scored 8 of 10<br />](https://learnah.org/wp-content/uploads/2018/08/Specimen_MS_-_Paper_3_AQA_Chemistry_A-Level_4-0.jpg)
