In this question you will determine the identity of the halogen in compound W. Compound W is the halogenoethanoic acid CH2XCO₂H, where X is a halogen. 4g of W were heated with 250 cm³ of 0.400 moldm-³ aqueous sodium hydroxide. Some of the sodium hydroxide reacted with compound W. The solution that remained after this reaction is FA 3. By titrating FA 3 with hydrochloric acid, you will determine how much of the sodium hydroxide remained after reaction with W. You will then calculate how much sodium hydroxide had reacted and use this to determine the identity of X in CH2XCO2H. FA 3 is aqueous sodium hydroxide after reaction with W. FA 4 is 0.100 mol dm³ hydrochloric acid, HCl. bromophenol blue indicator

1. (a)Method • Fill the second burette with FA 4. • Rinse the pipette with distilled water followed by a little FA 3. • Use the pipette to transfer 25.0 cm³ of FA 3 into a conical flask. • Add a few drops of bromophenol blue indicator. • 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 certain 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 4 added in each accurate titration. I II III • From your accurate titration results, obtain a suitable value for the volume of FA 4 to be used in your calculations. Show clearly how you obtained this value. 25.0 cm³ of FA 3 required ................................. cm³ of FA 4.[3]
2. (b)(i)Calculate the number of moles of hydrochloric acid, FA 4, present in the volume calculated in (a). moles of HCl = ................................ mol Hence deduce the number of moles of sodium hydroxide present in 25.0 cm³ of FA 3. moles of NaOH in 25.0 cm³ FA 3 = ................................ mol[1]
3. (b)(ii)Calculate the number of moles of sodium hydroxide added to the 4 g of W. moles of NaOH added to 4g W = ................................ mol Calculate the number of moles of sodium hydroxide that remain after the reaction with compound W. moles of NaOH remaining after reaction with W = ................................ mol[1]
4. (b)(iii)Calculate the number of moles of sodium hydroxide that reacted with W. moles of NaOH that reacted with W = ................................ mol Hence calculate the number of moles of W that reacted with this number of moles of sodium hydroxide. moles of W that reacted = ................................ mol[1]
5. (b)(iv)Use your answer to (iii), and the mass of W used to make FA 3, to calculate the Mr of W. Mr of W = ................................[1]
6. (b)(v)W is a halogenoethanoic acid, CH2XCO₂H. Use your answer to (iv) to determine the identity of X. Explain how you reached your conclusion.[2]
7. (c)Apart from any inaccuracies in reading the volumes of solutions, suggest a significant source of error in this practical exercise. Explain how you could minimise this error.[1]
8. (d)State at what Mr value of W, closest to the one calculated in (b)(iv), you would have concluded that X was a different halogen. Mr value = ................................[1]