NYJC 2020 Prelim 9729 P2

[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. ......................................................................................................................... ......................................................................................................................... ..................................................................................................................... [1] (ii) Determine the solubility of lead(II) hydroxide, X in mol dm3. [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 ......................................................................... at the end-point of the titration ...................................................................... [2] (iv) Determine the amount of lead( II) hydroxide precipitated from the water sample. [1] (v) Determine the maximum concentration of Pb2+ left in the mixture at the end - point. [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. [2] (vii) To deduce whether Fe 2+ is a good indicator 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. [1] (viii) Suggest another reason why Fe2+ ions are a good indicator for this titration. ......................................................................................................................... ..................................................................................................................... [1]

5 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use (ix) When exc ess 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. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ..................................................................................................................... [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 o f TEL, suggest a reason for its viscosity. ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ......................................................................................................................... ..................................................................................................................... [2]

6 H2 Chemistry 9729/02 NYJC J2/20 PX [Turn Over For Examiner's Use (