ACJC H2 CHEM P2 Questions Prelim

2 © ACJC 2015 9647/02/Prelim/15 [Turn over 1 Planning For Examiner’s use Scientific studies have provided strong evidence that lowering salt intake is beneficial in reducing blood pressure. Salt contains 40 % sodium which is essential for the normal functioning of the body. However, when eaten in excess, sodium raises blood pressure especially in individuals who are sodium-sensitive. The amount of salt in potato chips can be determined by Mohr’s Method. It is a direct titration method in which the amount of chloride ions can be determined. The amount of sodium ions can be calculated assuming all the sodium ions are present as sodium chloride. In this method, the chloride -containing sample solution is titrated with a standard solution of silver nitrate and chromate(VI) io ns are added as the indicator. After all of the chloride ions from the solution are precipitated with the silver io ns, orange -red precipitate of silver chromate(VI) will be formed with any further addition of silver ions. The reactions are: Ag+ (aq) + Cl- (aq)  AgCl (s) 2Ag+ (aq) + CrO4 2- (aq)  Ag2CrO4 (s) (a) You are to plan an experiment to investigate the percentage of sodium chloride in a 5 g sample of potato chips. Give a step -by-step description of how you would carry out the experiment including:  how you would prepare a solution of sodium chloride from the 5 g sample of potato chips for titration;  the essential details of titration procedure;  the appropriate quantities of reagents used. Your apparatus should only consist of the standard items found in a school laboratory. In addition to the standard apparatus, you are also provid ed with the following materials:  0.10 mol dm-3 of silver nitrate;  10 % potassium chromate(VI). ……………………………………………………………………………….................. ……………………………………………………………………………….................. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… ……………………………………………………………………………….................. ………………………………………………………………………………..................

3 © ACJC 2015 9647/02/Prelim/15 [Turn over …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… ……………………………………………………………………………….................. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… [5] For Examiner’s use

4 © ACJC 2015 9647/02/Prelim/15 [Turn over (b) (i) In one such titration using the Mohr’s Method, the exact concentration of silver nitrate used was determined using commercially available standard solution of sodium chloride. 25.0 cm3 solutions of silver nitrate were titrated against 0.114 mol dm -3 of sodium chloride. The results are reported below: No. Volume of NaCl /cm3 1 22.90 2 27.20 3 22.80 Calculate the exact concentration of silver nitrate. For Examiner’s use (ii) A solution containing sodium chloride was made by adding 10 0 cm3 of deionised water to a 5 g sample of potato chips. Using the silver nitrate solution from (b)(i), it was found that 25.0 cm3 of the salt solution required 8.70 cm3 of silver nitrate for titration. Calculate the percentage of sodium chloride in the 5 g sample of potato chips. [4]

5 © ACJC 2015 9647/02/Prelim/15 [Turn over (c) Mohr’s Method using chromate(VI) ions as the indicator can only be conducted under near neutral condition. Explain why the titration cannot be conducted when the pH of the solution is: For Examiner’s use (i) too low; ..................................................................................................................... ……………………………………………………………………………………. ……………………………………………………………………………………. ……………………………………………………………………………………. (ii) too high. ..................................................................................................................... ……………………………………………………………………………………. ……………………………………………………………………………………. [3] [Total: 12 marks] 2 A student prepared benzoic acid, C 6H5COOH, by hydrolysis of methyl benzoate, C6H5COOCH3 using the following procedure: Step 1 : Dissolve 4 g of sodium hydroxide in water to make 50 cm 3 of an alkaline solution. Step 2: Add aqueous sodium hydroxide to 2.70 g of methyl benzoate in a 100 cm 3 round bottom flask and set up the apparatus for reflux. Step 3: Heat the mixture for 30 minutes. Step 4: Distil the mixture until no more organic liquid is distilled out. Step 5: Pour the solution from the flask into a beaker and add dilute sulfuric acid until solution is acidic. Step 6 : Filter the crystals obtained and recrystallise from hot water to obtain the benzoic acid. (a) (i) Name the functional group that reacts during this hydrolysis. ……………………………………………………………………………………. (ii) Give the equation for the hydrolysis of methyl benzoate with sodium hydroxide. [2]

6 © ACJC 2015 9647/02/Prelim/15 [Turn over (b) The student obtained 1.50 g of benzoic acid. For Examiner’s use (i) Calculate the amount of both reactants used and hence identify the limiting reactant. (ii) Calculate the percentage yield of benzoic acid obtained. [5] (c) Explain why the residue from the flask was a cidified in Step 5 before recrystallising. .............................................................................................................................. …………………………………………………………………………………………… …………………………………………………………………………………………… [1] (d) (i) Identify the organic liquid distilled out in Step 4. ……………………………………………………………………………………. (ii) Suggest a method to check the purity of benzoic acid after recrystallisation. ……………………………………………………………………………………. ……………………………………………………………………………………. .................................................................................................................... [2] [Total: 10 marks]

7 © ACJC 2015 9647/02/Prelim/15 [Turn over 3 This question is about the Contact process and its catalyst, V2O5. The Contact process is an important industrial method of manufacturing sulfuric acid. For Examiner’s use The first stage of the Contact process involves the combustion of sulfur dioxide to give sulfur trioxide. SO2 (g) + ½ O2