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