ACJC_H2_CHEM_P2_ANS Prelim

2 1 Planning (P) When a solute is added to two solvents, A and B, which do not mix, some of the solute dissolves in each of the solvents and an equilibrium is set up between the two solvents. It has been shown that for dilute solutions, at equilibrium the ratio of the two concentrations is a constant known as the Partition Coefficient, K, and it will remain a constant if the solute remains in the same molecular state in the two solvents. Concentration of solute in solvent A = K Concentration of solute in solvent B An experiment was conducted to verify the above observation. Ethanoic acid (solute) was shaken with two immiscible solvents (water and cyclohexane, (C 6H12)), so that the solute distributed itself between the two solvents. The concentrations of the two solutions were then determined so that the ratio [CH 3COOH]aq/[CH3COOH]S known as the Partition Coefficient, K can be calculated at a given temperature. Using apparatus which are found as standard items in a school laboratory, an experiment was carried out to determine the Partition Coefficient , K for ethanoic acid, CH 3COOH, between water and cyclohexane, (C6H12). The following steps were carried out: 1. 5.0 g of the sample was first dissolved in 50 cm 3 of water in a beaker and the aqueous solution was then transferred into a separating funnel. 2. 50 cm 3 of cyclohexane was then poured into t he separating funnel containing the aqueous solution, stoppered and shaken intermittently. 3. The concentration of ethanoic acid in the two layers was then determined by titration with 0.05 mol dm-3 NaOH. (a) (i) Outline how a fixed volume of the sample of the aqueous layer can be removed from the mixture in the separating funnel for titrimetric analysis. (Note: The organic layer is less dense compared with the aqueous layer.) Either • Place a finger on the open end of a 10.0 cm 3 pipette and insert it into the bottom aqueous layer. • Gently blow lightly into the pipette to dispel any organic layer that may have entered the pipette before withdrawing the required amount of sample Or • Drain the aqueous layer into one dry beaker and the organic layer into another • Pipette 10.0 cm3 of the aqueous layer into a conical flask [2] ACJC 2011 9647/02/Aug/11 [Turn over

3 (ii) Outline stepwise how the removed sample could be analysed by titration to determine the concentration of the solute that had dissolved in that layer. • Place the pipetted sample into a 250 cm 3 conical flask • Add a drop of phenolphthalein • Fill a burette with 0.05 mol dm-3 aqueous NaOH • Titrate the sample until a faint pink colour is obtained •