10 (B410U10-1) Examiner only 12. (a) Caroline was investigating the number of moles of water of crystallisation, x, in hydrated barium chloride, BaCl2.xH2O. She was told that x is a whole number. She followed an instruction sheet. • Weigh an empty crucible with its lid. • Add the hydrated salt to the crucible and weigh crucible, lid and salt. • Place the lid on the crucible and heat salt for 3 minutes. • Cool and reweigh the crucible, lid and contents. • Heat for another 2 minutes and cool and reweigh again. Caroline obtained the following results. © WJEC CBAC Ltd. Mass / g Crucible + lid 13.132 Crucible + lid + BaCl2.xH2O 15.051 Crucible + lid + contents (after 1st heating) 14.787 Crucible + lid + contents (after 2nd heating) 14.777 (i) Use the data to determine the value of x in the formula BaCl2.xH2O. You must show your working. [4] x = ............................... (ii) Why did the instructions say that the lid should be in place when the heating was carried out? [1] (B410U10-1) Turn over. 11 Examiner only (iii) Ethan said that Caroline’s method was inaccurate, even though she had carried out the experiment carefully and recorded all her results correctly. Suggest two ways in which Caroline could make her experiment more accurate. Explain your answers. [4] Suggestion 1 Suggestion 2 (iv) Caroline agreed that her experiment had been inaccurate but said that it gave the correct answer for x. Comment on why Caroline was correct and that accuracy need not be high in this experiment to determine the value of x. [1] © WJEC CBAC Ltd. 12 (B410U10-1) Examiner only (b) Caroline used the barium chloride as one of the reagents to identify the ions present in an aqueous solution W. Solution W contains only two ions. The reagents were added to small volumes of solution W and the following observations were made. © WJEC CBAC Ltd. Test Observation add aqueous sodium hydroxide no visible reaction add aqueous barium chloride white precipitate formed add dilute nitric acid vigorous effervescence seen (i) From these observations name one ion present in solution W. [1] ........................................................................................................ (ii) The observations allowed Caroline to eliminate some metal ions as being present in W. Suggest one metal ion that she eliminated. [1] ........................................................................................................ (iii) Write the ionic equation for the reaction between aqueous solutions of barium chloride and W. Include state symbols. [1] 13 (B410U10-1) Turn over. 13 Examiner only 13. A group of students was given a mineral sample that came from a region where both magnesite, MgCO3, and dolomite, CaMg(CO3)2, were known to exist. They decided to analyse the sample by titration. They added 4.77 g of the mineral to 100 cm3 of 2.06 mol dm−3 hydrochloric acid. They titrated 25.0 cm3 samples of the solution formed against 1.00 mol dm−3 sodium hydroxide. The results of these titrations are given in the table. © WJEC CBAC Ltd. Titration number 1 2 3 Final burette reading / cm3 23.20 24.50 23.00 Initial burette reading / cm3 0.10 1.10 0.00 Titre / cm3 .................................... .................................... .................................... (a) Explain why universal indicator is not used to show the end-point of a titration. [1] (b) Complete the table and use the data to calculate a mean titre suitable for use in the calculation involved in the analysis of the sample. [1] Mean titre = ................................................ cm3 (c) Calculate the number of moles of hydrochloric acid that reacted with the mineral sample. [4] n(HCl) = ................................................ mol 14 (B410U10-1) Examiner only (d) Complete the ionic equation for the reaction between carbonate ions and acid and hence calculate the number of moles of carbonate present in the mineral sample. [1] CO3 2– + ........... H+ CO2 + ................................................ n(CO3 2–) = ................................................ mol (e) Calculate the relative formula mass of the carbonate and hence state whether the mineral is magnesite or dolomite. Assume that the mineral is a pure compound. [1] Mr = ................................................ (f) A burette can be read to an accuracy of ± 0.05 cm3. Calculate the maximum percentage error in any of the titres in the table. [2] Percentage error = ................................................ % © WJEC CBAC Ltd. (B410U10-1) Turn over. 15 Examiner only (g) The students extended their investigation by measuring the volume of carbon dioxide released when a 4.59 g sample of the mineral reacted with an excess of acid. They collected 1.31 dm3 of gas measured at 25 °C and at 1.01 × 105 Pa. (i) Use the ideal gas equation, pV = nRT, to calculate the number of moles of carbon dioxide formed. Show your working. [3] n(CO2) = ................................................. mol (ii) Show whether or not this extension confirms the conclusion reached in part (e). [1] END OF PAPER © WJEC CBAC Ltd. 14 Examiner only (B410U10-1) 16 © WJEC CBAC Ltd. For continuation only.