SAJC H2 Chem 2013 Prelim P2 Soln

1   St Andrew’s Junior College Suggested Answer Scheme Prelim 2013 Paper 2 1 (a) Preparation of saturated solution of lead iodate for titration  Stir and add solid lead iodate(V) to about 100 cm 3 of distilled water in a 250 cm3 beaker until no more solid dissolves.  Filter and collect the filtrate which contains saturated Pb(IO3)2 solution. Titration  Pipette 25.0 / 10.0 cm 3 of this saturated solution into a 250 cm 3 conical flask  Add excess solid K I to the flask. Use a 10 cm3 measuring cylinder to add about 5 cm3 of HCl (aq) to the flask.  Titrate the saturated solution with Na 2S2O3 from the 50 cm 3 burette into the flask  When the brown colour of the iodine fades to a pale yellow colour, add about 5 drops of starch solution to the flask.  Continue adding Na 2S2O3 until the blue-black colour just disappears.  Repeat the titrations to obtain at least two consistent results within ± 0.10 cm3.  Sequence [4] (b) One possible error could be any of the following statements (or more):  Adding the starch indicator too early in the experiment.  Temperature is not constant which might affect the solubility. [1] (c) Let the average titre of Na2S2O3 be v cm3. Amount of Na2S2O3 needed = 2 × 10-5 v mol Amount of iodine liberated = 1 × 10-5 v mol Amount of IO3 - = 3.33× 10-6 v mol [IO3 -] = 1.33 × 10-4 v mol dm-3 [Pb2+] = 6.67 × 10-5 v mol dm-3 Ksp = [Pb2+][IO3 -]2 = (1.33×10-4 v) 2 ( 6.67 × 10-5 v) = 1.179 × 10-12 v3 mol3 dm-9 [3] (d) Step Expected Observations Deduction To 1 cm3 of the mixture in a test tube, add dropwise of AgNO 3(aq) until in excess to the solution. White precipitate and yellow precipitate was formed. AgCl(s) Ag I(s) Add dropwise in excess NH3(aq). White precipitate dissolved in excess NH 3(aq); Yellow precipitate remained insoluble. [Ag(NH 3)2]+Cl-(aq) AgI(s) Filter the mixture and wash the residue with distilled water. Dry the residue by pressing between filter papers. Yellow residue Colourless filtrate obtained. Ag I(s) Cl-(aq) 1174

2   To the filtrate from above, add dropwise of HNO3(aq) in excess. Filter. Wash the residue with distilled water. Dry the residue by pressing between filter papers White residue obtained. AgCl(s) [4] [Total: 12 marks]   2. (a) (i) The VDW interaction / H - bonding between isopentyl acetate and water is not exothermic enough to overcome the hydrogen bonding between water and the id-id between isopentyl acetate which is predominant due to its large electron cloud size. [2] (ii) Isopentyl alcohol is a simple covalent molecule can form hydrogen bonding. Isopentyl acetate is a simple covalent molecule which can form id-id and pd-pd. Isopentyl acetate has more electrons which cause the id-id interactions to be more significant than hydrogen bonding in isopentyl alcohol. Hence more energy is required to overcome the intermolecular f