ACJC H2 CHEM P2 Answers Prelim

2 © ACJC 2015 9647/02/Prelim/15 [Turn over 1 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, t he chloride -containing sample solution is titrat ed with a standard solution of silver nitrate and chromate(VI) ions are added as the indicator. After all of the chloride ions from the solution are precipitated with the silver ions, orange-red precipitate of silver chromate(VI) will be formed with any fu rther addition of silver ions. The reactions are: Ag+ (aq) + Cl- (aq)  AgCl (s) 2Ag+ (aq) + CrO4 2- (aq)  Ag2CrO4 (s) [5] (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 provided with the following materials:  0.10 mol dm-3 of silver nitrate;  10 % potassium chromate(VI). 1. Weigh accurately 5 g of crushed potato chips into a 250 cm3 beaker. 2. Add 100 cm3 boiling water to the beaker. 3. Stir the mixture vigorously for 1 min and let it cool to room temperature. 4. Filter the solution and transfer the filtrate into a 250 cm 3 volumetric flask. 5. Top up to the mark with deionised water. Shake the volumetric flask to obtain a homogenous solution. 6. Pipette 25.0 cm3 of the filtrate to 250 cm3 conical flask. 7. Add 1 ml of potassium chromate indicator. 8. Titrate the solution with 0.10 mol dm -3 AgNO3 from the burette to the first visible orange-red colour. 9. Record the volume of titrant used. 10. Repeat the tit ration to obtain another consistent reading within ±0.10 cm3.

3 © 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. [4] Average volume of NaCl = 2 80.2290.22  = 22.85 cm3 Amount of AgNO3 = Amount of NaCl = 1000 22.85 x 0.114 = 2.60 x 10-3 mol Concentration of AgNO3 = 1000 25 1060.2 3 = 0.104 mol dm-3 (ii) A solution containing sodium chloride was made by adding 100 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. Amount of NaCl = Amount of AgNO3 = 8.70 x 0.104/1000 = 9.048 x 10-4 mol Mass of NaCl in 25.0 cm3 of solution = 9.048 x 10-4 x 58.5 = 0.0529 g Mass of NaCl in 5 g of NaCl = 0.0529 x 100/25 = 0.212 g Percentage of NaCl in 5 g of potato chips = %1005 212.0  = 4.23 %

4 © 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: [3] (i) too low; If the pH is low, chromate(VI) wil l be converted to dichromate . The chromate ion concentration is too low to produce the precipit ate at the equivalence point. (ii) too high. If the pH is high, brownish white precipitate of silver hydroxide forms and masks the end point. [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 ob tained and recrystallise from hot water to obtain the benzoic acid. [2] (a) (i) Name the functional group that reacts during this hydrolysis. Ester (ii) Give the equation for the hydrolysis of methyl benzoate with sodium hydroxide. Or C6H5COOCH3 + NaOH C6H5COONa + CH3OH (b) The student obtained 1.50 g of benzoic acid. + NaOH + CH3OH

5 © ACJC 2015 9647/02/Prelim/15 [Turn over [5] (i) Calculate the amount of both reactants used and hence identify the limiting reactant. Amount of methyl benzoate used = 15 3212 5 6)x 12( 7.2  = 0.0199 mol Amount of NaOH used = 4/40 = 0.100 mol NaOH is in excess and methyl benzoate is the limiting reactant. (ii) Calculate the percentage yield of benzoic acid obtained. theoretical yield of benzoic acid = 0.0199 x 122 = 2.43 g percentage yield = 1.50/2.43 x 100 = 61.7 % Alternative method: students use amount to calculate percentage yield (c) Explain why the residue from the flask was acidified in Step 5 before recrystallising. With alkaline hydrolysis, acidification is needed to obtain benzoic acid from its carboxylic acid salt Or to protonate/convert C6H5COO- to C6H5COOH [1] (d) (i) Identify the organic liquid distilled out in Step 4. Methanol [2] (ii) Suggest a method to check the purity of benzoic acid after recrystallisation. Measure the melting point of benzoic acid, it will melt at a fixed temperature if it is pure. Or melt at a small temperature range [Total: 10 marks]

6 © 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