10 (A410U10-1) Examiner only 10. The green mineral atacamite contains three different ions, Ma(OH)bXc. It is insoluble in water but can form a solution when added to acid. A student planned to analyse a sample of atacamite using the following method. STEP 1: Add 1.00 g of atacamite to 150 cm3 of strong acid of concentration 0.100 mol dm−3, and then make this up to exactly 250 cm3 with more of the same acid. STEP 2: Take exactly 25.0 cm3 of the solution from step 1 and add excess silver nitrate solution. If any precipitate forms, filter it off and dry completely. Record its colour and the mass formed. STEP 3: Take exactly 25.0 cm3 of the solution from step 1 and add excess barium chloride solution. If any precipitate forms, filter it off and dry completely. Record its colour and the mass formed. STEP 4: Take exactly 25.0 cm3 of the solution from step 1 and add excess sodium hydroxide solution. If any precipitate forms, filter it off and dry completely. Record its colour and the mass formed. STEP 5: To find the amount of acid remaining in the solution, use a pH probe to find the precise pH of the solution prepared in step 1. (a) The student had to select an appropriate strong acid from the common laboratory reagents. Suggest an appropriate acid to use. Explain your choice. [2] (b) In step 2 a white precipitate of mass 0.0672 g was produced. State which ion this step identifies and calculate the number of moles of this ion present in the original 250 cm3 of solution. [3] Ion ....................................................... Number of moles = ....................................................... mol © WJEC CBAC Ltd. (A410U10-1) Turn over. A 410 U101 11 11 Examiner only (c) After completing step 2, the student decides that he does not need to carry out step 3. Is he correct? Give a reason for your answer. [2] (d) A pale blue precipitate is formed in step 4. (i) Give the formula of the ion identified from this observation. [1] ....................................................... (ii) When the student heats the precipitate to ensure it is dry, a colour change is seen with some of the solid turning black but most of it remaining blue. He did not record a mass for the sample. Suggest why he would not have been able to use the mass of the solid to calculate the number of moles of the ion in the original compound. [1] © WJEC CBAC Ltd. 12 (A410U10-1) Examiner only (e) In step 5, the pH of a 25.0 cm3 portion of the remaining solution was found to be 1.36. (i) Calculate the number of moles of acid remaining in 25.0 cm3 of the solution. [2] Number of moles = ........................................................ mol (ii) Calculate the number of moles of hydroxide ion present in the original 1.00 g of atacamite. [3] Number of moles = ........................................................ mol (f) Deduce the formula of atacamite. [3] Formula of atacamite ................................................................................... © WJEC CBAC Ltd. 17 BLANK PAGE (A410U10-1) Turn over. 13 A 410 U101 13 © WJEC CBAC Ltd. (A410U10-1) 14 Examiner only 11. (a) Selenium is an element in the p-block of the Periodic Table. What information regarding the electronic structure of the selenium atom can be deduced from this statement? [1] (b) The illustration below gives some information about the elements surrounding selenium in the Periodic Table. © WJEC CBAC Ltd. S Sulfur electronegativity = 2.58 boiling temperature = 445 first ionisation energy = 1000 standard electrode potential for S(+6) → S(+4) = +0.16 V As Arsenic electronegativity = 2.18 boiling temperature = 603 first ionisation energy = 947 Se Selenium electronegativity = ? boiling temperature = 685 first ionisation energy = ? standard electrode potential for Se(+6) → Se(+4) = ? Br Bromine electronegativity = 2.96 boiling temperature = 60 Te Tellurium electronegativity = 2.10 boiling temperature = 990 standard electrode potential for Te(+6) → Te(+4) = +1.18 V All ionisation energies are given in kJ mol−1 and all temperatures in °C 15 (A410U10-1) Examiner only (i) Both sulfur and selenium have molecules containing rings of eight atoms. Explain the difference in their boiling temperatures. [2] (ii) Suggest values for the missing properties of selenium. Use the ideas you have studied to explain the values you have chosen. [6 QER] © WJEC CBAC Ltd. Turn over. A 410 U101 15 (A410U10-1) 16 Examiner only (c) One radioactive isotope of selenium is selenium-75. It can be used as a medical tracer to identify cartilaginous tumours. (i) The half-life of selenium-75 is 120 days. Samples are provided that have eight times higher concentration of selenium-75 atoms than the minimum needed for use as a tracer. Find the maximum time a sample can be stored before the concentration of selenium-75 becomes too low to use. [2] Time = ....................................................... days (ii) Selenium-75 does not emit alpha particles when it decays. Explain why this is important for its use as a medical tracer. [2] (d) Selenium dioxide, SeO2, is a foul smelling solid, with a smell resembling rotting horseradish. It can be used to oxidise alkenes. SeO2 + C3H6 + H2O H2SeO2 + C3H6O In an oxidation experiment, 2.70 g of C3H6 produced a yield of 62 % of C3H6O. Calculate the mass of C3H6O formed. [2] Mass = ....................................................... g © WJEC CBAC Ltd. 15