A-LevelChemistryAtoms, molecules and stoichiometry9701/31 · May/June 2024 · Q214 Marks
Sodium sulfite is oxidised when it reacts with excess iodine. Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2HI(aq) The remaining iodine is then titrated using aqueous sodium thiosulfate. I2(aq) + 2Na2S2O3(aq) → Na2S4O6(aq) + 2NaI(aq) You will determine the integer value of x in the formula of hydrated sodium sulfite, Na2SO3•xH2O, by titration. FA 4 is aqueous sodium thiosulfate containing 14.24 g of Na2S2O3 in 1.00dm³. FA 5 is aqueous iodine, prepared as shown. • 5.00 g of hydrated sodium sulfite is added to 600 cm³ of 0.100 mol dm¯³ aqueous iodine. • The mixture is allowed to stand to ensure that all the sodium sulfite has been oxidised. • The mixture containing the remaining iodine is made up to 1.00 dm³ with distilled water. FA 6 is starch indicator.
- (a)Method • Fill the burette with FA 4. • Pipette 25.0 cm³ of FA 5 into a conical flask. • Add FA 4 from the burette into the conical flask until the colour of the solution changes to yellow. • Add 10 drops of FA 6 to the conical flask. Continue titrating until the blue-black colour just disappears. • Perform a rough titration and record your burette readings in the space below. The rough titre is ................................... cm³. • Carry out as many accurate titrations as you think necessary to obtain consistent results. • Make sure any 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 4 added in each accurate titration.[7]
- (b)From your accurate titration results, calculate a suitable mean value to use in your calculations. Show clearly how you obtain the mean value. 25.0 cm³ of FA 5 required ............................................. cm³ of FA 4.[1]
- (c)(i)Give your answers to (c)(ii), (c)(iii), (c)(iv) and (c)(v) to an appropriate number of significant figures.[1]
- (c)(ii)Calculate the amount, in mol, of sodium thiosulfate present in the volume of FA 4 in (b).[1]
- (c)(iii)Calculate the amount, in mol, of iodine in 1.00 dm³ of FA 5.[1]
- (c)(iv)Use the information given and your answer to (c)(iii) to calculate the amount, in mol, of iodine that reacted with sodium sulfite when solution FA 5 was prepared.[1]
- (c)(v)Use your answer to (c)(iv) to calculate the relative formula mass, Mr, of hydrated sodium sulfite.[1]
- (c)(vi)Calculate the value of x. Show your working.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.
- [7] I The following data are recorded: • two burette readings AND titre for the rough titration • initial and final burette readings for two (or more) accurate titrations.
- II Titre values recorded for accurate titrations AND correct headings and units in the accurate titration table. • initial / start AND (burette) reading / volume • final / end AND (burette) reading / volume • titre OR volume used / added / OR FA 4 used / added • unit: / cm³ OR (cm³) OR in cm³ (for each heading) OR cm³ unit given for each volume recorded.
- III All accurate burette readings recorded to 0.05 cm³.
- IV The final accurate titre recorded must be within 0.10 cm³ of any other accurate titre.
- [Accuracy marks] Check and correct titre subtractions. Select the 'best' titres using the hierarchy (ignoring the rough titre value): 2 identical titres, 2 titres within 0.05 cm³, 2 titres within 0.10 cm³, etc. Calculate candidate's mean titre to 2 decimal places. Calculate the difference δ between the candidate's titre and the supervisor's titre.
- V Award if δ < 0.60 cm³
- VI Award if δ < 0.40 cm³
- VII Award if δ < 0.20 cm³
- [1] Candidate calculates mean titre correctly to 2 decimal places • 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 decimal places and be rounded to the nearest 0.01 cm³.
- [1] All answers given for (c)(ii), (iii), (iv) and (v) given to 3 or 4 significant figures.
- [1] Correctly calculates amount of sodium thiosulfate used = 14.24/158.2 × mean titre/1000 (mol)
- [1] Correct use of 2(c)(ii) amount of iodine (in 1 dm³) = (c)(ii)/2 × 1000/25 (mol)
- [1] Correct use of 2(c)(iii) amount of iodine reacting with sodium sulfite = 0.0600 – (c)(iii)
- [1] Correct use of 2(c)(iv) Mr of sodium sulfite = 5.0/(c)(iv)
- [1] Correct use of 2(c)(v) x = [(c)(v) – 126.1]/18 answer must be correctly rounded to an integer
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