12 (A410U10-1) Examiner only 12. Radioisotopes of elements are often used in the study of biological molecules. One such radioisotope is fluorine-18. (a) Give the numbers of protons and neutrons in the nucleus of a fluorine-18 atom. [1] Protons ……………....................………………… Neutrons ……………....................………………… (b) This radioisotope decays to form oxygen-18 only. Identify the type of radiation that must be emitted for this change to occur and identify any other type(s) of radiation that may also be emitted at the same time. [2] (c) A sample of fluorine gas contains four 18F atoms for every 19F atom. This was used to produce difluoromethanol, CHF2OH. A mass spectrum was taken as soon as the difluoromethanol had been synthesised. Part of the mass spectrum is shown below. © WJEC CBAC Ltd. 66 67 68 Abundance m/z (i) Identify the species that gives rise to the peak at m/z 66. [1] (A410U10-1) Turn over. 13 Examiner only © WJEC CBAC Ltd. (ii) The half-life of the fluorine-18 isotope is 110 minutes, and the original fluorine sample contained four fluorine-18 atoms for every fluorine-19 atom. Find the time taken to synthesise the difluoromethanol. [4] Time taken = …………..................…………………… minutes (d) The electronic structure of the oxygen atoms produced in this decay process may be studied by measuring successive ionisation energies. (i) Sketch a diagram showing the successive ionisation energies for oxygen. Show all eight ionisation energies. [2] 1 2 3 4 5 6 7 8 Number of electrons removed log (Ionisation energy) (ii) Explain how this diagram gives information regarding the position of the element in the Periodic Table. [2] 12 A 410 U101 13 14 (A410U10-1) Examiner only © WJEC CBAC Ltd. 13. Brass is an alloy of copper and zinc only. The copper content of the alloy can be found by volumetric or gravimetric analysis. The brass is dissolved by adding highly acidic mixtures to the alloy which forms Cu2+(aq) and amphoteric Zn2+(aq). (a) Redox titration is one method to find the mass of copper in a known mass of alloy. A 2.877 g sample of alloy is dissolved in concentrated nitric acid. The mixture is neutralised and then made up to a volume of 250.0 cm3. Samples of the solution with a volume of 25.00 cm3 are removed and excess potassium iodide solution added, before titration with 0.105 mol dm–3 sodium thiosulfate solution. The mean volume of sodium thiosulfate needed to completely reduce the iodine in solution is 26.75 cm3. Calculate the percentage by mass of copper in this alloy. You must show your working. [4] Percentage copper = ........................................................ % (A410U10-1) Turn over. 15 © WJEC CBAC Ltd. Examiner only (b) An alternative method is gravimetric analysis. Another sample of alloy is dissolved in concentrated nitric acid. The solution is neutralised and aqueous sodium hydroxide is added until all the copper(II) and zinc(II) ions form metal hydroxide precipitates. This sample is then filtered, dried and weighed (weighing 1). The solid sample is then treated with excess aqueous sodium hydroxide and the remaining solid is removed by filtration, dried and weighed (weighing 2). The results are given below. Mass of empty vessel = 23.34 g Mass of vessel and precipitate (weighing 1) = 25.12 g Mass of vessel and precipitate (weighing 2) = 24.45 g Calculate the percentage by mass of copper in this alloy. You must show your working. [4] Percentage copper = ........................................................ % (c) A student suggests that the alloys in parts (a) and (b) are the same. State and explain whether the evidence supports this statement and suggest what further evidence should be collected to confirm your conclusion. [2] 16 (A410U10-1) Examiner only © WJEC CBAC Ltd. (d) (i) Concentrated nitric acid is used to dissolve the alloy in the experiments above. The pH of this strong acid is typically –1.2. Calculate the concentration of this nitric acid. [2] Concentration = …………...................……………………….. mol dm–3 (ii) The acidic solution is neutralised using aqueous sodium hydroxide of concentration 2.00 mol dm–3. Calculate the pH of this sodium hydroxide solution. [2] [ionic product of water, Kw = 1.00 × 10–14 mol2 dm–6] pH = …………...................……………………….. 14 (A410U10-1) Turn over. 17 © WJEC CBAC Ltd. BLANK PAGE