CIE Jun 2015 v1 Paper 5 Q1

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A saturated aqueous solution of magnesium methanoate, Mg(HCOO)2, has a solubility of approximately 150 g dm3 at room temperature. Its exact solubility can be determined by titrating magnesium methanoate against aqueous potassium manganate(VII). During the titration, the methanoate ion, HCOO, is oxidised to carbon dioxide while the manganate(VII) ion, MnO4 , is reduced to Mn2+. You are supplied with: a saturated aqueous solution of Mg(HCOO)2 aqueous potassium manganate(VII), KMnO4, of concentration 0.0200 mol dm3 (a) (i) Write the half equations for the oxidation of HCOO(aq) to CO2(g) and the reduction of MnO4 (aq) to Mn2+(aq) in acid solution.[2] (ii) Using the approximate solubility above, calculate the concentration, in mol dm3, of the saturated aqueous magnesium methanoate and the concentration of the methanoate ions present in this solution. [Ar: H, 1.0; C, 12.0; O, 16.0; Mg, 24.3] [2] (iii) In order to obtain a reliable titre value, the saturated solution of magnesium methanoate needs to be diluted. Describe how you would accurately measure a 5.0 cm3 sample of saturated magnesium methanoate solution and use it to prepare a solutionfty times more dilute than the saturated solution.[2]UCLES 2015 9701/51/M/J/15<br /> (iv) Before the titration is carried out, dilute sulfuric acid must be added to the magnesium methanoate. Explain why this is necessary and also whether the volume of sulfuric acid chosen will affect the result of the titration.[2] (v) The potassium manganate(VII) is added from a burette into the magnesium methanoate in a conicalask. Describe what you would see when you had reached the end-point of the titration.[1] (vi) 1 mol of acidi ed MnO4ions reacts with 2.5 mol of HCOO ions. 25.0 cm3 of the diluted solution prepared in (iii) required 25.50 cm3 of 0.0200 mol dm3 potassium manganate(VII) solution to reach the end-point. Use this information to calculate the concentration, in mol dm3, of HCOO ions in the diluted solution. (vii) Use your answer to (vi) to calculate the concentration, in mol dm3, of the saturated solution of magnesium methanoate, Mg(HCOO)2. Give your answer to three signi cantgures.mol dm3 [1]UCLES 2015 9701/51/M/J/15 [Turn overmol dm3 [1]<br /> (b) The solubility of magnesium methanoate can be determined at higher temperatures using the same titration. In an experiment to determine how the concentration of saturated magnesium methanoate varies with temperature, name the independent variable and the dependent variable. independent variabledependent variable[1] (c) The solubility of magnesium methanoate increases with temperature. What does this tell you about H for the process below? Mg(HCOO)2(s) Mg2+(aq) + 2HCOO(aq) Explain your answer.[2] (d) A student used the same titration method, this time to measure the concentration of a saturated solution of barium methanoate. Explain why the acidi cation of the solution with dilute sulfuric acid might make the titration dif cult to do.[1] [Total: 15]UCLES 2015 9701/51/M/J/15<br /> QUESTION 2 STARTS ON THE NEXT PAGE.UCLES 2015 9701/51/M/J/15 [Turn over<br />

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