ASR 2020 J2Prelim H2Chem P4 MS

Anderson Serangoon Junior College 2020 JC2 H2 Chemistry Paper 4 Suggested Mark Scheme 1 Determination of the oxidation number of iodine in a compound In this experiment you will determine the oxidation number of iodine in one of its compounds by titration. You are provided with the following: FA 1 is a 0.0197 mol dm–3 solution of the iodine–containing compound. FA 2 is dilute sulfuric acid, H2SO4. FA 3 is aqueous potassium iodide, KI. FA 4 is 0.105 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator FA 1 reacts with excess acidified potassium iodide to produce iodine, I2. This iodine is then titrated with aqueous sodium thiosulfate using starch indicator. (a) Titration of iodine formed against FA 4 1. Fill the burette with FA 4. (You will use the same burette for Question 2.) 2. Pipette 25.0 cm3 of FA 1 into a conical flask. 3. Using a 25 cm3 measuring cylinder, add 10 cm3 of FA 2 to the same conical flask. 4. Using the same measuring cylinder, add 20 cm3 of FA 3 to the mixture in the conical flask. The mixture will now be a red–brown colour, due to iodine produced. 5. Add FA 4 from the burette until the mixture becomes light brown. 6. Then add 10 drops of starch indicator. The mixture will change to a dark blue colour. 7. Continue titrating until the mixture becomes colourless. This is the end–point. 8. Carry out as many accurate titrations as you think necessary to obtain consistent results. 9. Record in a suitable form below all of your burette readings and the volume of FA 4 added in each accurate titration. (i) Results 1 2 initial burette reading / cm3 0.00 10.00 final burette reading / cm3 28.40 38.25 volume of FA 4 / cm3 28.40 28.35 [5]

(ii) From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. 28.35 28.40 2  = 28.38 cm3 The iodine produced required 28.38 cm3 of FA 4. [1] (b) Show your working and appropriate significant figures in the final answer to each step of your calculations. (i) Calculate the number of moles of sodium thiosulfate in the volume of FA 4 calculated in (a)(ii). n(S2O32–) used = 28.380.105 x mol1000 = 2.98 x 10–3 mol moles of Na2S2O3 = ............................ [1] (ii) The equation for the reaction of iodine with sodium thiosulfate is shown. I2(aq) + 2Na2S2O3(aq)  Na2S4O6(aq) + 2NaI(aq) Calculate the number of moles of iodine that reacted with the sodium thiosulfate calculated in (i). n(I2) reacted = ½ x 2.98 x 10–3 = 1.49 x 10–3 mol moles of I2 = ............................ [1] (iii) Use the information on page 2 to calculate the number of moles of iodine – containing compound in the 25 cm3 of FA 1 used in each titration. FA1 25n = x 0.01971000 = 0.000493 mol moles of iodine–containing compound FA 1 = ........................