ACJC_H2_CHEM_P2_Answers Prelim

2 © ACJC 2014 9647/02/Prelim/14 [Turn over Planning 1 Distribution coefficient, KD, 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 distribution coefficient of acetylsalicylic acid (ASA) in two immiscible mixtures, water and trichloromethane as well as water and methylbenzene. The value s of the dis tribution coefficient can be used to determine Which is a better organic solvent for extracting ASA from water. It is predicted that each of these organic solvents will extract about 80- 90% of ASA dissolved in water. Distribution coefficient, KD (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 ASA (aq). 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 extracti ng 10.0 cm3 of the solution from the aqueous layer and titrating the aliquots against NaOH (aq). 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 cm3 of 0.01 mol dm-3 ASA (aq)  80 cm3 trichloromethane  80 cm3 methylbenzene  100 cm3 separating funnel for mixing and separating the two solutions  0.010 mol dm-3 stock solution of NaOH (aq) For Examiner’s use (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]

3 Independent variable – Organic solvent Dependent variable – Concentration of ASA in water and the organic solvent (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, s uggest why the 0.010 mol dm -3 NaOH stock solution is too concentrated for titrating the aqueous sample. [1] After extracting about 8 0-90% of ASA from the aqueous solution, the concentration of ASA (aq) is around 0.001 mol dm -3. The volume of 0.010 mol dm -3 NaOH used in the titration would be too small resu