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) → S₄O‰²¯(aq) + 21-(aq) FA 1 is hydrated sodium thiosulfate, Na2S2O3•xH2O. FA 3 is 0.0500 mol dm⁻³ iodine, I2. starch indicator

1. (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.[8]
2. (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]
3. (c)(i)Give your answers to (c)(ii) and (c)(iii) to the appropriate number of significant figures.[1]
4. (c)(ii)Calculate the number of moles of iodine in 25.0cm³ of FA 3. moles of I₂ = ................................... mol[1]
5. (c)(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]
6. (c)(iv)Calculate the value for x in the formula of hydrated sodium thiosulfate, Na2S2O3•XH2O. Show your working. x = ...................................[3]
7. (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]
8. (d)(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]
9. (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]
10. (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]