HSC_Chem_Past_Paper_Sol

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Unformatted text preview: al dissolved solids in the creek sample. Therefore any solids colled by filtration are not dissolved and therefore not total dissolved solids. Only the mass left behind after evaporation is of total dissolved solids. $\frac {45.59 - 45.33}{500} \times 100 = 0.052% \frac {w}{v}$ (b) Precipitation. Add iodine ions. The lead and iodine ions will form a bright yellow precipitate. $Pb^{2+} + I^- \rightarrow PbI_{2 \left ( s \right )}$ OR Atomic Absorbsion Spectroscopy (AAS). Where the substance is placed in a flame and the emmision spectra is either observed by the human eye or by a machine. Each metal ion has its own 'signature' emmision spectra. _____________________________________________________________________________________________________ HSC CHEMISTRY PAST PAPER SOLUTIONS – ANDREW HARVEY 11 2000 HSC ––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––– (c) Lead ions in waterways need to be monitored. If lead is present in drinking water, even at low concentrations, can be harmful to humans. Also lead in non-drinking waterways may need to be monitored to ensure that the marine life will not be affected adversly. Question 26: Sources of Contamination: *Farm/Vegetable Patch - Pesticides, fertelisers, other chemicals, animal droppings and decomposing organic matter may be washed into the lake when it rains. This could contaminate the lake water with toxic chemicals (eg. presticides) and the decaying organic matter could raise the biochemical oxygen demand (BOD). *Boats - Dirt and algue/weeds, etc from the bottom of the boat (that could have came from other rivers) could fall into the river and contaminate it. The dirt could raise the turbidity and total dissolved solids (TDS) of the water and the algue could be deadly or dangerous to humans to drink (eg. ecoli bacteria). Purifying Methods: *Screening - Removes large solid objects (eg. large branches, dead animals, rocks, etc.). *Sand Filtration - Removes smaller objects (eg. dirt) *Chemical Treatment - eg. Chlorine is added to kill bacteria, and fluride is added to strenghten teeth of people drinking the water. *Microscopic membrane filter - Can remove very fine particles from the water. Question 27: (a) Equlibrium has been reached. (b) (i) (I'm not sure if the two overlap as shown.) (ii) The spike in molar concentration at T<sub>2</sub> is because of the decrease in volume. As $c = \frac {n}{v}$, an decrease in ''v'' results in an increase in ''c''. This is the spike, however then Le Chatelier's prinicple kicks in. A decrease in volume results in an increase of pressure. As the reaction is $N_{2 \left ( g \right )} + 3H_{2 \left ( g \right )} \leftrightarrow 2NH_{3 \left ( g \right )}$ the total moles on the left is 4 and 2...
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