Edexcel Jan 2016 (IAL) Paper 3 Q3

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3 Washing soda is hydrated sodium carbonate, Na2CO3.xH2O, where the number of moles of water of crystallization, x, can vary. A sample of washing soda is analysed. Two methods are used to determine the value of x in the sample. Method 1: Heating 2.50 g of the washing soda is placed in a crucible. The crucible is gently heated for three minutes and then heated strongly for five minutes. The mass of the solid after heating is 1.06 g. Na2CO3.xH2O(s) (cid:111) Na2CO3(s) + xH2O(g) Equation 1 D O N O T W R I T E I N T H I S A R E A (a) Suggest why the crucible is heated gently for the first three minutes. (1)(b) What additional step after heating strongly for five minutes is needed to make sure that all of the water of crystallization has been removed? (1)(c) What is the correct chemical term for sodium carbonate without water of crystallization? (1)(d) (i) Calculate the number of moles of sodium carbonate that remain after heating the sample, assuming that all of the water of crystallization has been removed. (ii) Calculate the number of moles of water lost from the sample of washing soda on heating. *P46939A0620* (2) (1) D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> A E R A S I H T N I E T I R W T O N O D (iii) Hence deduce the value of x in the sample of washing soda, Na2CO3.xH2O, obtained using Method 1. (1) x =A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D *P46939A0720* Turn over<br /> Method 2: Titration Sodium carbonate reacts with hydrochloric acid as follows: Na2CO3 + 2HCl (cid:111) 2NaCl + CO2 + H2O Equation 2 A 2.50 g sample of the washing soda is placed in a beaker and dissolved in deionized water. This solution is poured into a 250 cm3 volumetric flask, made up to the mark and mixed thoroughly. A pipette is then used to transfer 25.00 cm3 of the washing soda solution to each of three conical flasks. A burette is filled with hydrochloric acid, of concentration 0.100 mol dm3, and titrations are carried out. The results are shown in the table. D O N O T W R I T E I N T H I S A R E A Titration numbers Burette reading (final) / cm3 Burette reading (initial) / cm3 Titre / cm3 17.00 0.00 17.00 33.55 17.00 16.55 16.45 0.00 16.45 (e) What should be done to make sure that all of the washing soda is transferred to the volumetric flask? (1)(f) Explain why only titrations 2 and 3 are used to calculate the mean titre. (1)*P46939A0820* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> (g) (i) Calculate the mean titre, and then calculate the number of moles of hydrochloric acid in the mean titre. (ii) Using your answer to part (g)(i) and Equation 2, calculate the number of moles of sodium carbonate present in 25 cm3 of the washing soda solution. (iii) Hence calculate the total number of moles of sodium carbonate present in 250 cm3 of the washing soda solution. A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D (iv) Calculate the molar mass of the hydrated washing soda, Na2CO3.xH2O. Hence deduce the value of x in the sample of washing soda from the data in Method 2. A E R A S I H T N (1) (1) (1) (2) I E T I R W T O N O D *P46939A0920* Turn over x = <br /> (h) A student carrying out Method 2 overshot the end-point of each titration. Explain how this would affect the calculated value of x. (2)(Total for Question 3 = 16 marks) 10 *P46939A01020* D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A D O N O T W R I T E I N T H I S A R E A<br /> A E R A S I H T N I E T I R W T O N O D A E R A S I H T N I E T I R W T O N O D BLANK PAGE A E R A S I H T N I E T I R W T O N O D *P46939A01120* 11 Turn over<br />

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