The partition coefficient, Kpc, shows the distribution of a solute between two immiscible solvents. Kpc is determined by measuring the concentration of the solute in each solvent. The organic solvent ethoxyethane, CH3CH2OCH2CH3, and water are immiscible. A student is asked to find Kpc of butanedioic acid, HOOCCH2CH2COOH, between ethoxyethane and water. The expression for Kpc when butanedioic acid is in equilibrium between ethoxyethane and water is shown. Kpc = [HOOCCH2CH2COOH(ethoxyethane)] [HOOCCH2CH2COOH(aq)] [density: ethoxyethane, 0.71gcm¯³; water, 1.00gcm-³] The student uses the following method to find the partition coefficient. A diagram of the apparatus is shown in [Figure 1.1]. step 1 Add 30.0cm³ of distilled water to a separating funnel. step 2 Weigh by difference 2.81 g of butanedioic acid into the separating funnel. step 3 Stopper the separating funnel and shake it until the butanedioic acid has dissolved. step 4 Remove the stopper and add 30.0cm³ of ethoxyethane to the separating funnel. step 5 Replace the stopper and shake the separating funnel gently. step 6 Place the separating funnel into a clamp. Allow the liquids to settle so that the two layers can be seen. step 7 Remove the stopper and open the separating funnel tap to allow the lower layer to run into a beaker labelled A. Run the upper layer into a beaker labelled B. step 8 Transfer 10.0cm³ of the aqueous layer into a conical flask. Titrate with 0.500moldm-3 NaOH(aq). Use thymolphthalein as the indicator. step 9 Take 10.0cm³ of the ethoxyethane layer and add 10.0cm³ of water to it. Titrate this mixture with 0.100 moldm¯³ NaOH(aq). Use thymolphthalein as the indicator.

1. (a)(i)State whether beaker A in step 7 contains the aqueous layer or the ethoxyethane layer. Explain your answer. Beaker A contains the ........................................................... layer. explanation ................................................................................................................................................................................................................................................................................................[1]
2. (a)(ii)Identify the piece of apparatus that should be used in step 8 to transfer 10.0cm³ of the aqueous layer.[1]
3. (a)(iii)Suggest why water is added to the ethoxyethane layer in step 9 before the titration can take place.[1]
4. (b)(i)For a 2.81 g sample of butanedioic acid, the titre for the aqueous layer is 27.25cm³ and the titre for the ethoxyethane layer is 22.50 cm³. The equation for the reaction between butanedioic acid and sodium hydroxide is shown. HOOCCH2CH2COOH + 2NaOH → NaOOCCH2CH2COONa + 2H2O Calculate the concentration of butanedioic acid in the aqueous layer. concentration of butanedioic acid = .............................................. moldm-3[1]
5. (b)(ii)Calculate the partition coefficient, Kpc. Kpc = ..............................................................[2]
6. (b)(iii)Explain why the student is only able to repeat the titration in step 8 once.[1]
7. (b)(iv)Suggest how you would modify the procedure to ensure the student can repeat the titration in step 8 more than once.[1]
8. (b)(v)A different student forgets to shake the separating funnel in step 5. Describe the effect this would have on the calculated Kpc value. Explain your answer. effect on Kpc ................................................................................................................................................................................................................................................................................................ explanation ................................................................................................................................................................................................................................................................................................[1]

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