4 2 This question is about sodium carbonate. (a) Sodium carbonate forms a number of hydrates with the general formula Na2CO3.xH2O. A 250 cm3 standard solution of one of these hydrates contained 10.0 g of the compound. Describe, including the names of any relevant apparatus, how to make this standard solution when provided with 10.0 g of the hydrate in a beaker. (5) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... Turn over 5 (b) 25.0 cm3 portions of the standard solution described in (a) are titrated with hydrochloric acid solution of concentration 0.300 mol dm–3, using methyl orange as an indicator. The table shows the results for this titration. Titration 1 Titration 2 Titration 3 Final volume / cm3 30.25 29.75 31.25 Initial volume / cm3 0.30 0.90 2.60 Total titre / cm3 29.95 28.85 28.65 (i) What is the colour change at the end-point of the reaction? (1) From To A red orange B red yellow C yellow orange D yellow red (ii) State why the value for the total titre in Titration 1 should not be used to calculate the mean titre. (1) .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... .................................................................................................................................................................................................................................................................................... (iii) Calculate the mean titre. (1) 6 (iv) Calculate the relative formula mass, Mr , of the hydrated sodium carbonate, Na2CO3.xH2O. The equation for the reaction in the titration is Na2CO3 + 2HCl → 2NaCl + H2O + CO2 (4) (c) In an experiment, the Mr of a different hydrated sodium carbonate was found to be 286 g mol–1. (i) Calculate the relative formula mass of anhydrous sodium carbonate, Na2CO3 . (1) (ii) Calculate the number of molecules of water of crystallisation, x, for this hydrated sodium carbonate, Na2CO3.xH2O. (1) Turn over 7 (d) Sodium carbonate is manufactured from sodium chloride in a two-stage process. NaCl + NH3 + CO2 + H2O → NaHCO3 + NH4Cl 2NaHCO3 → Na2CO3 + H2O + CO2 Calculate the maximum mass of sodium carbonate, Na2CO3 , which could be obtained from 500 kg of sodium chloride. (3) (Total for Question 2 = 17 marks)