Eduqas Chemistry for A Level Year 2: Student Bk

93 Exam practice questions (i) State the identity of the violet vanadium-containing solution produced in this reduction. Use standard electrode potentials to explain your answer. [3] (ii) What is the standard potential of a cell formed from a standard Zn 2+ (aq) / Zn(s) electrode and a standard Cu 2+ (aq) / Cu(s) electrode? [1] (iii) Write the equilibrium equation for the change occurring at the zinc electrode showing the direction in which the reaction proceeds. [1] (iv) Use Le Chatelier’s principle to predict the effect on the electrode potential of the zinc electrode of increasing the concentration of Zn 2+ (aq) in the electrode. Explain your answer. [2] 1 Write an ion-electron half-equation for the reduction of dichromate (VI) ions in acid solution, stating any colour change. [2] 2 Acidified potassium manganate(VII) can be used to oxidise ethanedioate ions, C 2 O 4 2– . Use the two ion-electron half-equations below to write an ionic equation for the reaction. [1] MnO 4 – + 8H + + 5e – Mn 2+ + 4H 2 O 2CO 2 + 2e – C 2 O 4 2– 3 An alloy containing iron was analysed by redox titration. A sample of 0·190 g of the alloy was converted to a solution of iron(II) ions, Fe 2+ , by dissolving in acid. The solution required 26·80 cm 3 of a solution containing dichromate(VI) ions, Cr 2 O 7 2– , of concentration 0·0162 mol dm –3 , for oxidation of all the Fe 2+ to iron(III) ions, Fe 3+ . (a) The ion/electron half–equation for the oxidation of iron(II) is Fe 3+ (aq) + e – Fe 2+ (aq) Write the ion/electron half-equation for the reduction of the oxidising agent Cr 2 O 7 2– in acid solution. [1] (b) Calculate the percentage by mass of iron in the alloy to an appropriate number of significant figures. [4] 4 Arsenic is a toxic element that in the past has been used in the form of the oxide As 4 O 6 as a poison. Analysis of a sample containing arsenic oxides amongst other materials was undertaken by attempting to dissolve 50.0 g in water, which dissolved the arsenic oxide leaving the remainder as an undissolved solid. (a) Give a method for separating the solution from the undissolved solids. [1] (b) The solution containing the arsenic was made up to 250 cm 3 and samples were titrated against a standard solution containing iodine. Arsenic(III) oxide reacts with iodine according to the following equation, giving an acidic solution. As 4 O 6 (aq) + 4I 2 (aq) + 4H 2 O(l) 2As 2 O 5 (aq) + 8HI(aq) (i) 20·0cm 3 of a standard iodine solution of concentration 0·0500 mol dm –3 reacted with 25·0 cm 3 of the arsenic containing solution. Calculate the number of moles of arsenic (III) oxide, As 4 O 6 , present in 25·0 cm 3 of the solution. [2] (ii) Use the answer to part (i) to calculate the total number of moles of arsenic(III) oxide, As 4 O 6 present and hence the percentage, by mass, of arsenic(III) oxide in the original sample of mass 50·0 g. Give your answer to an appropriate number of significant figures. [3] PI1.2