ACJC H2 CHEM P2 Prelim

2 1 Planning (P) For Examiner’s use 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 the 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.) [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. [2] For Examiner’s use (b) When analysing the organic layer, a fixed volume of water (about twice the volume) is added to the pipetted volume of cyclohexane solution in the conical flask and shaken vigorously, prior to the titration. Explain the purpose of this action. [2] (c) Based on your procedure, state and explain one significant error or limitation that could be encountered in determining an accurate value for the distribution ratio. [1] (d) Suggest one possible modification that would minimise the error or limitation that you have stated in (c). Explain how this modification leads to an improvement in the accuracy and reliability of the results. [1] ACJC 2011 9647/02/Aug/2011 [Turn over

4 (e) A student did the above experiment but carried out only one titration using a fixed aliquot of the aqueous layer and then proceeded to use the titre value to calculate a value for K. Assuming the student obtained a titre value of y cm 3 of M mol dm-3 NaOH (aq) when 10.0 cm3 of the aqueous layer was titrated, outline how the student could have used the result to determine a value for K. [3] For Examiner’s use (f) When a solute is distributed between the two immiscible solvents, it is important to ensure that the resulting solutions are dilute for the distribution to occur and the partition coefficient to remain constant. Suggest why this is critical in this experiment. [1] [Total: 12] 2 (a) A hydrocarbon W contains 92.3 % carbon. On complete combustion, 0.005 mol of W produces 1.76 g of CO2. (i) Determine the molecular formula of W. ACJC 2011 9647/02/Aug/11 [Turn over

5 (ii) Given that 1 mol of W reacts with 1 mol of bromine in tetrachloromethane, draw the displayed formula of W. For Examiner’s use [3] (b) Apart from using aqueous bromine or bromin e in an inert solvent, describe one other simple chemical test you could carry out to distinguish W from O COOH State clearly how each compound behaves in the test and write an equation for the reaction involved. Reagent(s) & conditions: Observations: Equation: [3] ACJC 2011 9647/02/Aug/2011 [Turn over

6 (c) Complete the reaction scheme below by writing the structural formula of the organic products (A-C) and the reagents and conditions (1-4) in the spaces below. For Examiner’s use O COOH 3 C H3 B A 2 CN C H3 O O NH CH 3 4 C room temp. C H3 Br OH 1 PCl5 [7] [Total: 13] ACJC 2011 9647/02/Aug/11 [Turn over

7 3 (a) Nitrogen and phosphorus belong to Period 2 & 3 respectively. At 373 K, both phosphorus trichloride (boiling point = 350 K) and nitrogen trichloride (boiling point = 344 K) exist as a gas. For Examiner’s use (i) State one assumption in the kinetic theory of gases that causes the deviation of a real gas from ideal behaviou