VJC 2021 Prelim H2P4 Ans [FInal]

1 Ó VJC 2021 9729/04/PRELIM [Turn over Victoria Junior College 2021 H2 Chemistry Prelim Exam 9729/4 Suggested Answers 1 Determination the extent of oxidation of iron(II) solution exposed to air Read through the whole method before starting any practical work. Where appropriate, prepare a table for your results in the space provided. Show your working and appropriate significant figures in the final answer to each step of your calculations. It is essential to dissolve solid samples of iron(II) compounds in acids such as dilute sulfuric acid instead of water. This is to prevent excessive oxidation of iron(II) ions to iron(III) ions in the presence of oxygen over a period of time. FA 1 is 0.020 mol dm–3 acidified potassium manganate(VII), KMnO4. FA 2 is 1.0 mol dm–3 sulfuric acid, H2SO4. FA 3 is iron(II) salt dissolved in water and left standing for an extended period of time. Using the apparatus and chemicals provided, you are required to determine the extent of oxidation of iron(II) salt in FA 3. (a) Dilution of FA 3 1. Pipette 25.0 cm3 of FA 3 into a 100 cm3 volumetric flask. 2. Make up to the contents of the flask to the 100 cm3 mark with deionised water. Shake to obtain a homogeneous solution. 3. Label this solution diluted FA 3 which is to be used for (b) and (c). (b) Determination of amount of iron(II) ions originally present in FA 3. 1. Add 50 cm3 of diluted FA 3 using a measuring cylinder into a 100 cm3 beaker. 2. Add about 2 spatula of zinc powder into the beaker and stir. 3. Filter the mixture into a boiling tube. 4. Pipette 10.0 cm3 of the filtrate into a conical flask. 5. Add 10 cm3 of FA 2 into the conical flask using a measuring cylinder. 6. Titrate this solution against FA 1 from a burette until the first permanent pale pink colour is obtained. 7. Record your titration results, to an appropriate level of precision, in the space below. You are to perform this titration only once. Results: For both (b) and (c) table Correct headers with units; Tabulation of ALL burette readings to 2.d.p. Experiment 1 Final burette reading / cm3 19.00 Initial burette reading / cm3 0.00 Volume of FA 1 used / cm3 19.00 [2]

2 Ó VJC 2021 9729/04/PRELIM [Turn over (c) Determination of amount of iron(II) remaining in FA 3. 1. Pipette 10.0 cm3 of diluted FA 3 into a conical flask. 2. Add 10 cm3 of FA 2 into the conical flask using a measuring cylinder. 3. Titrate this solution against FA 1 from a burette until the first permanent pale pink colour is obtained. 4. Record your titration results, to an appropriate level of precision, in the space below. 5. Repeat the titration as many times as necessary to obtain consistent results. Results: Experiment 1 2 Final burette reading / cm3 13.00 13.00 Initial burette reading / cm3 0.00 0.00 Volume of FA 1 used / cm3 13.00 13.00 At least two consistent readings ±0.10 cm3 Accuracy [2m] [3] (d) Calculations (i) From your titration results in