DHS_H2_CHEM_P2_ANSWERS Prelim

© DHS 2011 9647/02/Answers 1 Dunman High School 2011 Year 6 H2 Chemistry Preliminary Exams – Paper 2 Answers 1 Planning When aqueous sodium chloride, NaC l, is added to aqueous lead( II) nitrate, Pb(NO 3)2, a white precipitate of lead chloride, PbCl2, is produced. A suggested stoichiometric equation is Pb(NO3)2(aq) + 2NaCl(aq)  PbCl2(s) + 2NaNO3(aq) In separate experiments, different volumes of 0.20 mol dm –3 aqueous sodium chloride are added to a fixed volume of 0.10 mol dm –3 aqueous lead( II) nitrate. In each case, the precipitate, PbCl2 will be formed. This precipitate is then filtered, washed with distilled water and thoroughly dried. The mass of the precipitate is recorded. You are to plan an experiment to investigate this reaction in order to confirm or reject the stoichiometry of the equation. (a) (i) By considering the suggested stoichiometric equation, predict how the number of moles of the precipitate, PbC l2, will change as the number of moles of NaC l added increases. Prediction: No. of moles of PbCl2, increases as no. of moles of NaCl added Increases. (ii) Sketch the graph which would result if NaC l is the limiting reagent . Start your graph with no NaCl added. (b) You are provided with 250 cm 3 of 0.20 mol dm –3 aqueous sodium chloride and some solid lead(II) nitrate. (i) Outline how you would prepare 250 cm 3 of 0.10 mol dm –3 aqueous lead( II) nitrate. [Ar: N, 14.0; O, 16.0; Pb, 207.0] • Weigh 8.275 g of solid Pb(NO3)2 into an empty weighing bottle / beaker. • Transfer and dissolve it in some distilled/deionised water in a beaker. • Rinse weighing bottle with distilled water and transfer the washings to the beaker. • Ensure all solid has dissolved. PbCl2 /mol NaCl /mol 0

© DHS 2011 9647/02/Answers 2 • Transfer to a 250 cm 3 standard/volumetric/graduated flask using a funnel and glass rod. • Rinse beaker with distilled water and transfer the washings to the flask • Add distilled water to the standard/volumetric/graduated flask to the 250 cm3 mark • Shake thoroughly to ensure a homogenous mixing. (ii) You will be carrying out five sets of experiments to obtain different masses of precipitate. Give a step by step description of how you would carry out one such experiment. You need to state only the following:  the volumes of each solution to be used,  how the volumes will be measured, and  how you would dry the precipitate. • Measure 40 cm 3 of Pb(NO 3)2 into an beaker using a measuring cylinder/burette/pipette • Measure 5 cm3 of NaCl using a measuring cylinder/burette/pipette and • Dry the precipitate by: o allowing water to evaporate o pressing between filter papers o warming in oven o adding propanone (c) In the table below  enter appropriate headings to show recorded and calculated data. The headings should include appropriate units,  enter the volumes from your plan in (b),  e