NYJC 2020 Prelim 9729 P2 Answers

[Turn over NANYANG JUNIOR COLLEGE JC 2 PRELIMINARY EXAMINATION Higher 2 CANDIDATE NAME CLASS TUTOR’S NAME 1 9 CENTRE NUMBER S INDEX NUMBER CHEMISTRY 9729/02 Paper 2 Structured 31 August 2020 2 hours Candidates answer on the Question Paper Additional Materials: Data Booklet READ THESE INSTRUCTIONS FIRST Write your Centre number, index number, name and class on all the work you hand in. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs. Do not use staples, paper clips, glue or correction fluid. Answer all questions in the spaces provided on the Question Paper. The use of an approved scientific calculator is expected, where appropriate. A Data Booklet is provided. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. For Examiner’s Use 1 /25 2 /10 3 /14 4 /11 5 /15 Total /75 This document consists of 23 printed pages and 1 blank page.

2 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use Answer all questions in the spaces provided. 1 Heavy metals can leach into water from taps inside older buildings due to the stagnation of water in tanks and water pipes inside the buildings. A study of the tap water from a building showed that it contained small amounts of lead(II) ions , when consumed past a certain limit, have toxicological effects on human beings. World Health Organisation (WHO) issues guidelines on the limits of various heavy metal ions in drinkable water. The WHO limit for lead( II) ions concentration is 5.0 108 mol dm3. An environment agency wants to determine if the tap water from the building is within the WHO limits. Through a precipitation titration, the concentration of Pb 2+ in the water sample can be determined. You may find the following information useful:  A titration was conducted where 1.00 d m3 of sample water was titrated against aqueous sodium hydroxide.  1.00  105 mol of iron(II) nitrate was used as the indicator.  At the end -point, the solid was filtered and the mass of lead( II) hydroxide precipitate was found to be 1.20  105 g.  The water sample only contains lead ions as contaminants. (a) (i) Explain the term solubility. The solubility of a salt refers to the maximum amount or mass that can dissolve in 1 dm 3 of solvent at a given temperature to form a saturated solution. [1] ......................................................................................................................... ..................................................................................................................... [1] (ii) Determine the solubility of lead(II) hydroxide, X in mol dm3. Pb(OH)2 (s) ⇌ Pb2+(aq) + 2OH(aq) (X)(2X)2 = 1.4 1020 X = solubility of lead hydroxide = 1.518  107 mol dm3 = 1.52 107 mol dm3 [1] [1] Precipitate Mr Colour of precipitate value of Ksp solubility / mol dm3 Pb(OH)2 241.2 white 1.4  1020 X Fe(OH)2 89.8 green 4.9  1017 2.30  106

3 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use (iii) Using the data given, state what you would observe at the beginning of the titration white ppt [1] ..................................................... at the end-point of the titration green ppt [1] ................................................. [2] (iv) Determine the amount of lead( II) hydroxide precipitated from the water sample. n(Pb2+) = 51 20 10 241 2 . .  = 4.98  108 mol [1] (v) Determine the maximum concentration of Pb2+ left in the mixture at t he end- point. At the end-point, 1st trace of Fe(OH)2, (Ionic product = Ksp) where [Fe2+] = 0.00001 mol dm3 [OH] = 174 9 10 0 00001 . .  = 2.213 106 mol dm3 [1] [Pb2+] = 20 62 1 4 10 2 213 10 . ( . )     = 2.857 109 mol dm3 = 2.86 109 mol dm3 [1] [2]

4 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use (vi) Hence, using your answers in (iv) and (v), determine the initial concentration of Pb 2+ in the water sample and deduce if the concentration is within the WHO limit. Initial n(Pb2+) = 4.975  108 + 92 857 10 1 00( . . )  = 5.260  108 mol Initial [Pb2+] = .  8 3 5 260 10 1 dm = 5.26 108 mol dm3 [1] ( > 5.0 108 mol dm3) The lead concentration is over the limit. [1] [2] (vii) To deduce whether Fe 2+ is a good ind icator in this titration, most of the lead(II) ions needs to be precipitated at the end-point. Show that more than 90% of the lead( II) ions in the water sample has been precipitated when the end-point was reached. At first trace of Fe(OH)2, most of the Pb2+ has precipitated. %n(Pb2+) ppted = .. % . %.       89 8 5 260 10 2 857 10 100 94 65 260 10 [1] OR Only a small amount of Pb2+ remained in solution. %n(Pb2+) left in solution = . % . %.     9 8 2 857 10 100 5 4305 260 10 %n(Pb2+) ppted = 100% - 5.430% = 94.6% [1] (viii) Suggest another reason why Fe2+ ions are a good indicator for this titration. The colour change from white to green is distinct. [1] ......................................................................................................................... ..................................................................................................................... [1]

5 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use (ix) When excess sodium hydroxide was added after the end -point, it was observed that the white precipitate dissolved. With the aid of relevant equations, explain the observations.  Pb(OH)2 (s) ⇌ Pb2+(aq) + 2OH(aq) ------------------- (1)  Pb(OH)2 (s) + 2OH(aq) ⇌ [Pb(OH)4]2(aq) ---------- (2)  When excess OH is added, Pb2+ reacts to form a soluble complex formed.  As [Pb2+] decreases, Ionic product of Pb(OH)2 < Ksp  POE (1) shifts to the right. Hence, ppt dissolves. [2] – 5 points [1] – 2-4 points ......................................................................................................................... ..................................................................................................................... [2] (b) Tetraethyl lead (TEL), an organometallic compound was the main anti-knock agent used in cars in the past. It is a dense, viscous (slow-flowing) liquid which is highly lipophilic - soluble in non-polar solvents such as petrol and hexane. Tetraethyl lead (TEL) (i) With reference to the structure and bonding of TEL, suggest a reason for its viscosity.  TEL has a simple molecular structure with instantaneous dipole-induced dipole forces between TEL molecules.  TEL has a large number of electrons per molecule OR large electron cloud therefore more easily polarised.  Hence, significant instantaneous dipole -induced dipole forces between TEL molecules leading to packing more molecules in a g iven volume/ more energy is required to break the id-id forces between the molecules. 3 pts – [2] 1-2 pts – [1] ......................................................................................................................... ...............................................................................................................