12 (B410U10-1) 12. Barium chloride exists as a hydrated salt, BaCl2.xH2O, where x is the number of molecules of water of crystallisation. To determine the value of x, the hydrated salt is heated to remove the water of crystallisation. The following method was carried out. • Weigh an empty crucible with its lid. • Place about 2.00 g of hydrated barium chloride in the crucible and re-weigh, with its lid. • Place the lid on the crucible and heat gently at first, then remove the lid and heat strongly for about 2 minutes. © WJEC CBAC Ltd. lid heat crucible • Place the lid on the crucible and allow it to cool. • Weigh the cooled crucible with its lid and residue. The following results were recorded. Mass of crucible + lid = 10.24 g Mass of crucible + lid + hydrated barium chloride = 12.25 g Mass of crucible + lid + residue after heating = 11.97 g (B410U10-1) Turn over. 13 Examiner only (a) Calculate the mass of hydrated barium chloride and the mass of residue. Hence determine the value of x. [5] x = ........................................................ (b) (i) Suggest why the crucible was heated initially with the lid in place. [1] (ii) Suggest why the lid was placed on the crucible when it was left to cool. [1] © WJEC CBAC Ltd. 14 (B410U10-1) Examiner only (c) (i) When this experiment is carried out the value of x obtained is often less than the theoretical value. Suggest a reason for this, assuming that the compound contains no impurities. [1] (ii) State an improvement that could be made to the practical procedure to overcome the problem you have identified in (c)(i). [1] (d) If the error in each balance reading is ± 0.05 g, calculate the percentage error in the mass of hydrated barium chloride used in the experiment. [1] Percentage error = ........................................................ % (e) Apart from the improvement you identified in (c)(ii), suggest another change that could be made to improve the accuracy of the experiment. [1] © WJEC CBAC Ltd. 11 (B410U10-1) Turn over. 15 © WJEC CBAC Ltd. BLANK PAGE TURN OVER FOR QUESTION 13