ACJC_H2_CHEM_P2_Questions Prelim

2 © ACJC 2014 9647/02/Prelim/14 Planning 1 Distribution coefficient, K D, is the ratio of equilibrium concentrations of a compound in a mixture of two immiscible solvents at a fixed temperature. Using two immiscible solvents, the distribution coefficient is a measure of the difference in solubility of a compound in these two solvents. This experiment aims to find out the dist ribution coefficient of acetylsalicylic acid (ASA) in two immiscible mixtures, water and trichloromethane as well as water and methylbenzene. The values of the distribution coefficient can be used to determine which is a better organic solvent for extracting ASA from water. It is predicted that each of t hese organic solvents will extract ASA from water with an efficiency of 80-90%. Distribution coefficient, K D (organic solvent / water) = [ASA] in organic solvent / [ASA] in water COOH O O Acetylsalicylic acid To find KD (organic solvent /water) of ASA, a certain volume of the organic solvent is added to a measured volume of 0.01 mol dm -3 aqueous ASA. Vigorous shaking of the mixture enables ASA to dissolve in the mixture of solvents. The mixture will reach equilibrium when it is left to stand for an hour at a suitable temperature. The concentration of ASA in each solvent is determined by extracting 10.0 cm 3 of the solution from the aqueous layer and titrating the aliquot against aqueous NaOH. By assuming that the remaining amount of ASA is dissolved in the organic solvent, the concentration of ASA in organic solvent can be calculated. You are provided with  80 cm 3 of 0.01 mol dm-3 ASA (aq)  80 cm 3 trichloromethane  80 cm 3 methylbenzene  100 cm 3 separating funnel for mixing and separating the two solutions  0.010 mol dm -3 stock solution of NaOH (aq) For Examiner’s use

3 @ACJC 2014 9647/02/Prelim/14 (a) In an experiment, the independent variable is a parameter that can be manipulated independently, whereas the dependent variable is the data to be collected that varies with the parameters that are manipulated. State the independent variable and dependent variable in this experiment. [2] For Examiner’s use (b) A student who was conducting the experiment discovered that the NaOH stock solution should be diluted to 0.001 mol dm -3 before it is suitable for titrating the aqueous sample. In light of the extraction efficiency of the organic solvents, suggest why the 0.010 mol dm -3 NaOH stock solution is too concentrated for titrating the aqueous sample. [1] (c) Procedure Describe the procedure to determine KD (trichloromethane/water) of ASA. Your plan should include  necessary steps to set up the equilibrium of ASA in water and trichloromethane  appropriate ap