A-LevelChemistryAtoms, molecules and stoichiometry9701/33 · Feb/Mar 2022 · Q215 Marks
Solid sodium sulfite is often provided as the hydrated salt, Na2SO3•xH2O, where x is an integer. You will determine x by using a solution of this sodium sulfite and reacting it with an excess of aqueous iodine. Na2SO3(aq) + I2(aq) + H2O(l) → Na2SO4(aq) + 2I⁻(aq) + 2H⁺(aq) The amount of iodine remaining will be determined by titration using a known concentration of sodium thiosulfate, Na2S2O3. I2(aq) + 2S2O3²⁻(aq) → 2I⁻(aq) + S4O6²⁻(aq) FA 3 is a solution containing 31.50 g dm⁻³ of hydrated sodium sulfite, Na2SO3•xH2O. FA 4 is 0.100 mol dm⁻³ iodine, I2. FA 5 is 0.100 mol dm⁻³ sodium thiosulfate, Na2S2O3. FA 6 is starch indicator.
- (a)Method • Pipette 10.0 cm³ of FA 3 into a conical flask. • Pipette 25.0 cm³ of FA 4 into the same flask. • Swirl the flask to mix the contents. • Fill the second burette with FA 5. • Add FA 5 to the flask until the mixture is yellow. • Add approximately 10 drops of FA 6. • Complete the rough titration by adding FA 5 until the mixture is colourless. 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 below, all of your burette readings and the volume of FA 5 added in each accurate titration. I II III IV V VI VII[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. 10.0 cm³ of FA 3 plus 25.0 cm³ of FA 4 required ...................................... cm³ of FA 5.[1]
- (c)(i)Give your answers to (c)(ii), (iii) and (iv) to an appropriate number of significant figures.[1]
- (c)(ii)Use your answer to (b) to calculate the amount, in mol, of sodium thiosulfate, FA 5, required to react with the excess iodine which remained in the conical flask. amount of Na2S2O3 = ...................................... mol Hence calculate the amount, in mol, of iodine, FA 4, remaining in the conical flask. amount of I₂ remaining = ...................................... mol[1]
- (c)(iii)Calculate the amount, in mol, of iodine, FA 4, added to the conical flask. amount of I2 added = ...................................... mol Hence calculate the amount, in mol, of iodine that reacted with the 10.0 cm³ of sodium sulfite, FA 3. amount of I₂ that reacted with Na2SO3 = ...................................... mol[1]
- (c)(iv)Use your final answer to (c)(iii) and the information on page 5 to calculate the amount, in mol, of sodium sulfite present in 1.00 dm³ of FA 3. amount of Na2SO3 in 1.00dm³ = ...................................... mol[1]
- (c)(v)Use your answer to (c)(iv) to calculate the value of x in Na2SO3•xH2O. x = ......................................[2]
- (d)A student suggests that sodium carbonate should be added to each mixture of sodium sulfite and iodine in the conical flask before titrating with sodium thiosulfate. State whether you agree with the student. Explain your answer.[1]
Mark scheme
Marking points from the official mark scheme (Feb/Mar 2022). The mark or mark type is shown in brackets.
- [1] I The following data must be shown: • burette readings and titre for rough titration • 2 × 2 'box' showing both accurate burette readings.
- [1] II Headings and units correct for accurate titration table and headings match readings. • initial / start AND (burette) reading / volume + unit • final / end AND (burette) reading / volume + unit • titre OR volume / FA 4 AND used / added + unit
- [1] III All accurate burette readings given to the nearest 0.05
- [1] IV The final accurate titre recorded is within 0.10 cm³ of any other accurate titre.
- [3] Accuracy marks Check and correct titre subtractions where necessary. Exclude any titre from the calculation for the mean where final burette reading is greater than 50(.00). Examiner selects the best titres for calculating the mean, using the hierarchy: 2 identical titres, 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 δ. Award V if δ < 0.50 cm³ Award VI if δ < 0.30 cm³ Award VII if δ < 0.20 cm³
- [1] Candidate must average two (or more) titres that are all within 0.20 cm³ AND give the answer to 2 dp. AND working must be shown or ticks must be put next to the two (or more) accurate titres selected.
- [1] All final answers for (c)(ii), (c)(iii), (c)(iv) are to 3–4 sf
- [1] Correctly calculates amount of (n) Na2S2O3 = 0.1 × (b) / 1000mol AND n(I2) = ans / 2
- [1] Correctly calculates initial amount of (n) I2 = 2.5(0) × 10⁻³ mol AND n(I2) that reacted = 2.5(0) × 10⁻³ – final answer to (ii)
- [1] Correctly uses amount of (n) Na2SO3 = final answer to (iii) × 100 mol
- [1] M1 Correctly uses Mr Na2SO3•XH2O = 31.5(0)/ (c)(iv) AND Mr Na2SO3 = 126.1
- [1] M2 n(H2O) = (answer above – 126.1)/18 AND answer is a (correctly rounded) integer OR M1 mass(Na2SO3) = (c)(iv) × 126.1 AND mass(H2O) = 31.5(0) – answer above M2 n(H2O) = mass above / 18 AND x = n(H2O)/n(Na2SO3) AND answer is a (correctly rounded) integer
- [1] agree: Na2CO3 would neutralise acid (formed in reaction between sulfite and iodine which would react with the thiosulfate) / some thio would (otherwise) react with the acid (formed) OR disagree: (no advantage) as the reaction of thio with acid is slow OR disagree: (no advantage) as Na2CO3 will react with iodine (so less to react with thio)
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