Question 119 marks
Question
In this experiment you will determine the value of x in the formula of hydrated sodium thiosulfate, Na2S2O3•xH2O, where x is an integer. You will first prepare a solution of the salt and then use this solution in a titration with aqueous iodine. The thiosulfate ions react with iodine as shown. 2S2O32–(aq) + I2(aq) → S4O62–(aq) + 2I–(aq) FA 1 is hydrated sodium thiosulfate, Na2S2O3•xH2O. FA 3 is 0.0500 mol dm–3 iodine, I2. starch indicator (a) Method Preparation of salt solution ● Weigh the container containing FA 1. ● Tip the contents of the container into the 250 cm3 beaker. ● Weigh the container with any residue. ● Record all your readings in the space below. ● Add approximately 200 cm3 of distilled water to the salt in the beaker and stir until the salt has dissolved. ● Pour the contents carefully into the 250 cm3 volumetric flask. ● Rinse the beaker with a little distilled water and add these washings to the flask. ● Fill the flask to the mark with distilled water and shake to ensure thorough mixing. ● Label this solution FA 2. Titration ● Fill a burette with FA 2. ● Pipette 25.0 cm3 of FA 3 into the conical flask. ● Add FA 2 from the burette until the solution in the flask turns yellow. ● Add 10 drops of starch indicator to the conical flask. The solution will turn blue-black. ● Continue to add more FA 2 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 below. The rough titre is …… cm3. ● Carry out as many accurate titrations as you think necessary to obtain consistent results. ● Make sure your recorded results show the precision of your practical work. ● Record, in a suitable form in the space below, all of your burette readings and the volume of FA 2 added in each accurate titration. I II III IV V VI VII VIII [8] (b) From your accurate titration results, obtain a value for the volume of FA 2 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm3 of FA 3 required …… cm3 of FA 2. [1] (c) Calculations (i) Give your answers to (c)(ii) and (c)(iii) to the appropriate number of significant figures. [1] (ii) Calculate the number of moles of iodine in 25.0 cm3 of FA 3. moles of I2 = …… mol [1] (iii) Calculate the number of moles of thiosulfate ions in the volume recorded in (b). moles of S2O32– = …… mol Hence calculate the number of moles of hydrated sodium thiosulfate in the mass weighed in (a). moles of Na2S2O3•xH2O = …… mol [1] (iv) Calculate the value for x in the formula of hydrated sodium thiosulfate, Na2S2O3•xH2O. Show your working. x = …… [3] (d) (i) State the maximum error in a single reading on the balance used in (a). maximum error = ± …… g Calculate the maximum percentage error in the mass of FA 1 used in (a). Show your working. maximum percentage error = ± …… % [1] (ii) Assume that the uncertainty in the mass of FA 1 is the only source of error in your experiment. Calculate the minimum value for the relative formula mass of FA 1. Show your working. minimum value for the relative formula mass of FA 1 = …… [1] (e) A student prepares FA 2 using anhydrous sodium thiosulfate salt and the same mass of salt that you used in (a). State how the student’s titre would compare with the average titre value you obtained in (b). Explain your answer. . …… . …… . …… . …… [1] (f) In many titrations it is usual to fill the burette with the solution of known concentration. Suggest why this was not done in (a). . …… . …… [1] [Total: 19]
Mark scheme
1(a) I Clearly shows initial and final mass and both recorded to the same precision and with correct units. 1 Units: (g) or / g or in g or g by every entry (including mass used if given) Reject weight II The following data must be shown 1 • burette readings and titre for rough titration • 2 × 2 ‘box’ showing both accurate burette readings III Headings and units correct for accurate titration table and headings match readings. 1 • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FA 2 and used / added Units: (cm3) or / cm3 or in cm3 or cm3 by every entry Allow vol for volume, value for reading Allow change in volume Reject difference, total or amount IV All accurate burette readings are recorded to 0.05 cm3 (including 0.00) 1 Reject if 50(.00) is used as an initial burette reading; Reject if more than one final burette reading is 50(.00) Reject if any burette reading is > 50(.00) V The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Ignore any titre labelled ‘rough’ Reject if any ‘accurate’ burette reading (apart from an initial 0 cm3) is given to zero dp. Check and correct titre and mass 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 calculates supervisor’s corrected average titre × supervisor’s corrected mass to 1 dp. Examiner calculates candidate’s corrected average titre × candidate’s corrected mass to 1 dp. Subtract the candidate product value from that of the supervisor: δ 1(a) Award VI if 4.0 < δ ⩽ 6.0 g cm3 1 Award VI and VII if 2.0 < δ ⩽ 4.0 g cm3 1 Award VI, VII and VIII if δ ⩽ 2.0 g cm3 1 If there is only one accurate titration award accuracy marks based on that titration without further penalty. If only a rough titration is shown award accuracy marks based on this value but cancel one accuracy mark. Apply spread penalty as follows:if titres selected (by examiner) differ ⩾ 1.00 cm3 then cancel one accuracy mark. If Supervisor’s value ⩽ 10.00 cm3 then halve tolerances (3 marks: δ ⩽ 1.0; 2 marks: δ ⩽ 2.; 1 mark: δ ⩽ 3.0 g cm3) 1(b) Candidate must average two (or more) titres that are all within 0.20 cm3. 1 Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1(c)(i) Answers for (ii) and both parts of (iii) are quoted to 3−4 sf. 1 1(c)(ii) Correctly calculates 1 1.25 × 10–3 1(c)(iii) Correctly uses 1 2.50 × 10−3 AND 250 0.625 2.50 × 10−3 × = (b ) (b ) 1(c)(iv) Correctly uses 1 correctly calculated mass from (a) ans (iii ) Display of ( ANS − 158.2 ) 1 18 1(c)(iv) Uses values to calculate x to the nearest integer 1 1(d)(i) Uncertainty in a single reading: 1 for 1 dp balance allow the uncertainty given to be ± 0.1 or 0.05 for 2 dp balance allow the uncertainty given to be ± 0.01 or 0.005, etc. AND 2 × uncertainty given Display of × 100 candidate mass 1(d)(ii) Correctly uses 1 (100 − ( d)(i ) ) × candidate’s Mr 100 OR (mass ( a ) − 2 × ( single ) uncertainty ) ans ( c )(iii ) Allow numerator as (mass (a) – single uncertainty) if lack of doubling already penalised in (d)(i). 1(e) Titre value is less because concentration of thiosulfate is greater 1 1(f) The indicator colour change is easier to see blue-black to colourless. 1 OR The dark colour of the aqueous iodine makes the burette harder to read.