Edexcel Jun 2016 Paper 4 Q12

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SECTION B Answer ALL the questions. Write your answers in the spaces provided. 12 Sulfuric acid, H2SO4, is a well known acid containing sulfur. However, two other sulfur-containing acids are hydrogen sulfide, H2S, and sulfurous acid, H2SO3. (a) Hydrogen sulfide is a weak acid and dissociates in two stages as shown. Stage 1 H2S(aq) + H2O(l) (cid:85) H3O+(aq) + HS(aq) Ka1 = 8.91108 mol dm3 Stage 2 HS(aq) + H2O(l) (cid:85) H3O+(aq) + S2(aq) Ka2 = 1.201013 mol dm3 D O N O T W R I T E I N T H I S A R E A (2) 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 Write the Ka expressions for Stage 1 Ka1 = Stage 2 Ka2 = 12 *P46660RA01228*<br /> (b) A solution of hydrogen sulfide has an initial concentration of 0.100 mol dm3. Ka1 = 8.91108 mol dm3 (i) Use Ka1 to calculate the equilibrium concentration, in mol dm3, of the hydrogensulfide ion, HS. Give your answer to three significant figures. (ii) Use your answer to (b)(i) to calculate the pH of this solution. *(iii) State the three assumptions you have made in your calculations in (b)(i) and (b)(ii). (2) (1) (3) 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 DA E R A S I H T N I E T I R W T O N O D *P46660RA01328* 13 Turn over<br /> (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (cid:3) (c) Sulfurous acid, H2SO3, is also a diprotic acid. The values of Ka1 and Ka2 can be determined from the results of an acid-base titration. Diprotic acids require two OH ions per molecule for complete neutralization. Sulfurous acid, H2SO3, is a stronger acid than H2S and a 0.100 mol dm3 solution has a pH of 1.5. (i) On the grid below, sketch the likely shape of the titration curve for sulfurous acid, H2SO3, during the neutralization process. (cid:404)(cid:3) 25 cm3 of sulfurous acid solution with a concentration of 0.100 mol dm3 is used (cid:404)(cid:3) (cid:3)100 cm3 of the sodium hydroxide solution with a concentration of 0.100 mol dm3 is added (cid:404)(cid:3) pKa1 = 1.9 and pKa2 = 7.2 Clearly label any equivalence points in the sketch. 14 pH 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 (5) (1) 25 50 75 100 Volume of sodium hydroxide added / cm3 (ii) Describe how you would use this graph to confirm the value of pKa1.(Total for Question 12 = 14 marks) 14 *P46660RA01428*<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 *P46660RA01528* 15 Turn over<br />

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