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 …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… ……………………………………………………………………………….................. …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………………………………………………………………… …………………………………