When concentrated iron(III) chloride is added to water at just below boiling point, a reaction occurs and produces Fe2O3, seen as a red colour in the water. This is a 'sol' of Fe2O3. A sol contains particles that are insoluble but do not form a precipitate. A student prepared a concentrated solution of iron(III) chloride by dissolving FeCl3.6H2O(s) in distilled water. [Figure showing U-tube with d.c. power supply, graphite electrodes, distilled water, and red Fe2O3 sol] Salt solutions can be added to sols to cause them to precipitate. This method is used in water purification. The student made up 100.0 cm³ of standard solutions containing 0.100 mol dm³ of the following ions. K+(aq) Mg2+(aq) Al3+(aq) Cl-(aq) SO42-(aq) PO43-(aq) The student carried out experiments to investigate how much of a particular salt solution was required to fully precipitate all the Fe2O3 sol in a 1000 cm³ sample. The salt solutions used were all of concentration 0.0100 mol dm³ with respect to the ion being investigated. [Figure table for Experiment 1 and Experiment 2 showing identity of salt solution, charge on anion/cation, and minimum amount required for complete precipitation]

1. (a)Hazard information for hydrated iron(III) chloride is given. For this hazard, state a precaution, other than eye protection and a lab coat, that the student could take when preparing a solution of concentrated iron(III) chloride. hazard: solid FeCl3.6H2O is irritating to the skin precaution[1]
2. (b)Particles of a sol can be positively or negatively charged. The student used the experimental set-up shown to confirm that the Fe2O3 sol particle is positively charged. The student placed a few cm³ of the sol at the bottom of the U-tube and poured 10 cm³ of distilled water into each side of the U-tube, without disturbing the sol. The two layers of distilled water were colourless at the beginning of the experiment. Graphite electrodes were inserted and a current was passed. After 30 minutes a difference was noted between the distilled water in the two sides of the U-tube. Predict the colour of the distilled water in both sides of the U-tube after 30 minutes, if the Fe2O3 sol particle is positively charged. observation in side with positive electrode observation in side with negative electrode[1]
3. (c)(i)What mass of solid potassium sulfate, K2SO4, did the student use to make up exactly 100.0 cm³ of 0.100 moldm¯³ SO42-(aq)? [Ar: K, 39.1; S, 32.1; O, 16.0] mass of K2SO4 =[1]
4. (c)(ii)Describe how the student should have accurately prepared this volume of standard solution from a sample of K2SO4 of mass calculated in (c)(i).[2]
5. (d)The student carried out an experiment to precipitate the Fe2O3 sol, using 0.100 mol dm-³ K2SO4(aq). Only one drop of K2SO4(aq) was needed for the complete precipitation of 10.0 cm³ Fe2O3 sol. Calculate how many moles of SO42- were added. Assume that one drop is 0.05 cm³. moles of SO42- added =[1]
6. (e)(i)Calculate the volume of standard solution required to make exactly 50.0 cm³ of 0.0100 mol dm³ K2SO4(aq). volume of standard K2SO4(aq) =[1]
7. (e)(ii)Name a piece of apparatus that could be used to measure accurately the volume of solution calculated in (e)(i).[1]
8. (f)In an alternative method, 50.0 cm³ of 0.0100 moldm¯³ K2SO4(aq) could be prepared by using 0.0872 g of K2SO4. Explain why the dilution method used by the student to prepare 50.0 cm³ of 0.0100 mol dm-³ K2SO4(aq) is the more accurate of the two methods.[1]
9. (g)(i)Describe the effect of changing the charge on the anion from −1 to −2 to -3 on the precipitation of the Fe2O3 sol in Experiment 1.[1]
10. (g)(ii)Identify the independent variable in Experiment 2.[1]
11. (g)(iii)Arsenic sulfide, As2S3, is highly toxic and should be removed from drinking water. The Fe2O3 sol particles are positively charged. The As2S3 sol particles are negatively charged. Based on the student's results, which salt used in either Experiment 1 or Experiment 2 would be the most effective at removing As2S3 from drinking water? Explain your answer. salt explanation[2]

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