2019 SAJC H2 Chem Prelim P4 ANS

[Turn Over H2 Chemistry SAJC Prelims Paper 4 Review 1 To standardise a 0.03 mol dm –3 of iron( III) chloride solution and determine the concentration of sodium thiosulfate solution. (a) Planning FA1 is a standard solution containing 0.03 mol dm–3 of FeCl3 solution. You are to design an experiment to show how FA1 is prepared using solid FeCl3. You are provided with : a solid sample of approximately 1.50 g of FeCl3 (Mr = 162.3), 250 cm3 volumetric flask and the usual laboratory apparatus In your plan, you should include details on:  the mass of FeCl3(s) you use with justification,  the apparatus you would use, and  the procedures which you would follow. Do not carry out this plan as FA1 is provided for you to continue with 1(b). To make standard solution, a 250 cm3 volumetric flask is used. Amount of FeCl3 in 250 cm3 = 0.03 x 250 / 1000 = 7.50 × 10–3 mol Mass of FeCl3 = 7.50 × 10–3 x 162.3 = 1.22 g Procedure: 1. Weigh accurately about 1.22 g (2 d.p) of FeCl3 into a dry clean weighing bottle. 2. Dissolve the solid using distilled water (~ 50 cm3) in a 100 cm3 beaker. 3. Transfer the solution after ensuring all the solid has dissolved. Rinse the weighing bottle and beaker with water and transfer all washings into a 250 cm 3 volumetric flask to ensure quantitative transfer 4. Top up the solution with distilled water to the 250 cm3 mark. 5. Stopper and shake well to obtain a homogenous solution. Label this solution FA 1. procedures 1-5. [3]

2 (b) In addition to FA1, you are also provided with the following: FA2 is 0.060 mol dm–3 potassium iodide, KI. FA3 is aqueous sodium thiosulfate, Na2S2O3. When FA1 is combined with FA3, a complex, [Fe(S2O3)2]–, is formed. Reaction 1: Fe3+(aq) + 2S2O32–(aq) ⇌ [Fe(S2O3)2]–(aq) To 1 cm3 of FA3 in a test–tube, add 2–3 drops of FA1 and shake them thoroughly. State the colour of the complex formed. (Pale) purple/ lilac /Pink/ violet. [1] (c) A student aimed to determine the concentration of FA3 using volumetric analysis involving the use of FA1 and FA2. Reaction 2: 2Fe3+(aq) + 2I–(aq) → 2Fe2+(aq) + I2(aq) Reaction 3: I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) Using the above reactions, the student planned and carried out the following procedure. 1. Fill a 50.00 cm3 burette with FA3 solution. 2. Using a 10 cm3 measuring cylinder, measure 5 cm3 of FA1 and place it in a 100 cm3 conical flask. 3. Using another 10 cm 3 measuring cylinder, measure 5 cm 3 of FA2 into the conical flask. 4. Run FA3 from the burette into the conical flask. Near the endpoint, when the brown solution becomes pale, add 2 drops of starch. 5. Continue adding FA3 slowly, the endpoint is reached when th e solution first becomes colourless. 6. Record the titration results, to an appropriate level of precision, in an appropriate table and repeat the experiment once more. (i) Carry out the student’s procedure twice only to obtain two titratio