Question 115 marks
Question
Bleach is made by reacting chlorine with a cold solution of sodium hydroxide. This reaction produces sodium chlorate(I), NaCl O. Cl 2(aq) + 2NaOH(aq) → NaCl O(aq) + NaCl (aq) + H2O(l) In this experiment you will determine the concentration of sodium chlorate(I) in a sample of bleach, FA 1. To do this, you will react an acidified dilute solution of the bleach with iodide ions, I–. This reaction produces iodine, I2. Cl O–(aq) + 2I–(aq) + 2H+(aq) → I2(aq) + Cl –(aq) + H2O(l) The amount of iodine produced will then be determined by titration with thiosulfate ions, S2O32–. I2(aq) + 2S2O32–(aq) → 2I–(aq) + S4O62–(aq) FA 1 is a solution of bleach. FA 3 is dilute sulfuric acid, H2SO4. FA 4 is 0.500 mol dm–3 potassium iodide, KI. FA 5 is 0.100 mol dm–3 sodium thiosulfate, Na2S2O3. starch indicator (a) Method Dilution ● Pipette 25.0 cm3 of FA 1 into the 250 cm3 volumetric flask. ● Add distilled water to make 250 cm3 of solution and shake the flask thoroughly. ● Label this flask FA 2. Titration ● Fill a burette with FA 5. ● Rinse the pipette thoroughly with distilled water and then with a little FA 2. ● Pipette 25.0 cm3 of FA 2 into a conical flask. ● Use the measuring cylinder to add 20 cm3 of FA 3 to the conical flask. ● Use the measuring cylinder to add 15 cm3 of FA 4 to the conical flask. The solution will turn brown as iodine is produced. ● Add FA 5 from the burette until the solution has turned yellow. ● Add 10 drops of starch indicator to the conical flask. The solution will turn blue‑black. ● Continue to add more FA 5 from the burette until the blue-black colour just disappears. This is the end‑point of the titration. ● Carry out a rough titration and record your burette readings in the space provided. The rough titre = …… cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure that your recorded results show the precision of your practical work. ● Record in a suitable form in the space below all your burette readings and the volume of FA 5 added in each accurate titration. Keep FA 4 and FA 5 for use in Question 3. I II III IV V VI VII [7] (b) From your accurate titration results, obtain a value for the volume of FA 5 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 2 required …… cm3 of FA 5. [1] (c) Calculations (i) Give your answers to (c)(ii) and (c)(iii) to the appropriate number of significant figures. [1] (ii) Use your answer to (b) and the relevant equation on page 2 to calculate the number of moles of iodine that formed when 25.0 cm3 of FA 2 reacted with FA 4. moles of I2 = …… mol [1] (iii) Calculate the concentration of sodium chlorate(I) in FA 1. Show your working. concentration of NaCl O = …… mol dm–3 [2] (d) (i) In this method an excess of potassium iodide must be added to 25.0 cm3 of FA 2. Use your answer to (c)(ii) to show by calculation that the potassium iodide is in excess. [1] (ii) A student carries out the same method but the concentration of the potassium iodide solution is not stated. What change to the practical procedure could the student make to check that the potassium iodide is in excess? Explain your answer. . …… . …… . …… [2] [Total: 15]
Mark scheme
1(a) l The following data must be shown: 1 • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings II Headings and units correct for accurate titration table and headings match readings. 1 • initial/start and (burette) reading / volume + unit • final/end and (burette) reading / volume + unit • titre or volume/FA 5 and used / added + unit Units: (cm3) or / cm3 or in cm3 or cm3 by every entry III All accurate burette readings to 0.05 cm3 1 1(a) IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre 1 Accuracy (Q) marks Round burette readings to the nearest 0.05 cm3. Check and correct titre subtractions where necessary. Examiner selects the best mean titre. Apply hierarchy: 2 identical, titres within 0.05 cm3, titres within 0.10 cm3 etc. Examiner subtracts (corrected) candidate’s titre from Supervisor’s titre. Write and ring supervisor’s value next to the accurate titration table of each candidate, also candidate mean value (calculated by examiner) and difference between supervisor’s and candidate’s value, δ. Award accuracy marks as follows: 7 V Award if δ ⩽ 0.60 cm3 VI Award if δ ⩽ 0.40 cm3 VII Award if δ ⩽ 0.20 cm3 (3) 1(b) Correctly calculates mean accurate titre 1 • Candidate must take the average of two (or more) titres that are within a total spread of not more than 0.20 cm3. • Working/explanation must be shown or ticks must be put next to the two (or more) accurate readings selected. • The mean should be quoted to 2 dp and be rounded to nearest 0.01 cm3. 1(c)(i) Answers for (c)(ii) and (iii) to 3 or 4 sf 1 1(c)(ii) Correctly calculates amount of I2 1 = [(b) × 0.1] / [2 × 1000] (= (b) × 5 × 10−5 mol) 1(c)(iii) Correctly uses: 2 If candidate’s answer = 400 × (c)(ii) and working shown award both marks If candidate’s answer = 400 × (c)(ii) no working shown award 1 mark If candidate’s answer = 200 × (c)(ii) award 1 mark (I2 / I− confusion) If candidate’s answer = 100 × (c)(ii) award 1 mark (250 / 25 instead of 1000 / 25) If candidate’s answer = 40 × (c)(ii) award 1 mark (dilution factor missing) 1(d)(i) moles of KI = 15 / 1000 × 0.5 = 7.5 × 10–3 mol 1 moles I− = (c)(ii) × 2 2 x (c)(ii) < 7.5 × 10–3 OR a comparison in words ((c)(ii) < 3.75 × 10–3) 1(d)(ii) M1: Repeat the experiment using a greater volume of KI 2 M2: If titre value is the same then KI was in excess OR M1: Add more KI at the endpoint M2: If mixture / it goes black / blue-black / dark blue then KI was not in excess OR M1: Add AgNO3 before titrating M2: Yellow ppt observed shows presence of I− / no yellow ppt shows I− not in excess OR M1: Add more/extra NaClO when the titration mixture is yellow M2: if the mixture goes darker yellow / brown etc., (this shows more iodine has been produced, as) KI is in excess. (ORA)