Year 12 Chemistry Tutorial 9.4.C – Ion Analysis Answers 1.
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sodium sulfate 
sodium hydroxide 
lead sulfate 
potassium phosphate 
magnesium sulfate 
sodium chloride 
calcium sulfate 
ammonium bromide 
barium sulfate 
silver chloride 
sodium carbonate 
silver nitrate 
lead iodide 
lead chloride 
calcium hydroxide 
silver iodide 
lead phosphate 
potassium sulfate 2.
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Lead nitrate – Pb(NO3)2 Sodium chloride – NaCl Silver nitrate and potassium carbonate AgNO3 (aq) + K2CO3 (aq)  Ag2CO3 (s) + KNO3 (aq) (a) 2Na3PO4 (aq) + 3PbNO3 (aq)  Pb3(PO4)2 (s) + 6NaNO3 (aq) (b) CaCl2 (aq) + Li2SO4 (aq)  CaSO4 (s) + 2LiCl (aq) (c) Ca(NO3)2 (aq) + NaOH (aq)  Ca(OH)2 (s) + NaNO3 (aq) (d) Mg(NO3)2 (aq) + 2NaOH (aq)  Mg(OH)2 (s) + 2NaNO3 (aq) (e) 2LiCl (aq) + Ag2SO4 (aq)  Li2SO4 (aq) + 2AgCl (s) (f) Sr(NO3)2 (aq) + MgSO4 (aq)  SrSO4 (s) + Mg(NO3)2 (aq) Some cations have the same colour precipitates with various anions. For example, calcium and barium form white precipitates with sulfates, phosphates and carbonates. To distinguish between calcium and barium ions we need to do a flame test where the calcium ions will produce a red colour and while barium ions will produce a green colour in the flame. Also, because the colour of some precipitates are similar, further tests may be needed to confirm the identify of an unknown. Iron phosphate is green while iron carbonate is grey. To confirm the carbonate, acid is added to dissolve the precipitate, producing carbon dioxide gas. When electrons are excited they jump into higher energy levels. When they return to the ground state they release energy as light. The frequency of the light will correspond to the difference in energy between the energy levels. Since strontium and calcium will produce different energy differences, the light they give of will have a different frequency. Since the colour of light depends on frequency, strontium and calcium will produce their own characteristic colour. (a) Barium ion 2+
2-­‐
(b) Ba (aq) + SO4 (aq)  BaSO4 (s) The addition of chloride ions would yield a white precipitate of PbCl2. If there were calcium ions present, no precipitate would form. 9.
(a) Carbonate ion 2+
2-­‐
2-­‐
+
(b) Fe (aq) + CO3 (aq)  FeCO3 (s) and CO3 (aq) + 2H (aq)  CO2 (g) + H2O (l) 2+
10. The addition of Fe ions would produce a green precipitate with nitrate ions but no precipitate with chloride ions. 11. (a) 67.5 % sulfate and 22.3 % sulfur (b) 17.5 % nitrogen 12. Qualitative analysis of ions refers whether or not they are present in a solution. A quantitative analysis would give a concentration of the ion in solution. 13. At low levels, the concentrations of many ions are harmless to humans and other animals. When they reach critical levels they can become toxic. The slow accumulation of some heavy metal ions in the food chain means that some seafood, for example, can contain dangerously high levels of undesirable ions. Some foodstuffs are monitored for the presence of particular metals, e.g. lead, to ensure that we are not consuming poisons that can accumulate in our bodies. The concentration of lead in our blood increases when we inhale air from busy roads. The concentration of lead in blood needs to be monitored, especially for populations of children living near busy roads and workers in lead smelters. Action can then be taken or evidence used to make changes, such as in the use of petrol additives or location of worksites or in work practices. 14. Each element has its own characteristic absorption spectrum that is related to its electron energy levels. 15. The flame containing the vaporised sample absorbs light at the particular wavelengths characteristic of the element in the flame and re-­‐emits it in all directions. A detector records the intensity of light emerging from the flame. The intensity of light detected drops sharply at the wavelengths of light absorbed by the elements in the flame, thus producing an absorption spectrum. The relative intensity and pattern of changes of intensity within each of the bands in the absorption spectrum are indicative of the concentration of the element in the test sample. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. A working curve is a plot of the analytical signal (the instrument or detector response) as a function of analyte concentration. These working curves are obtained by measuring the signal from a series of standards of known concentration. The working curves are then used to determine the concentration of an unknown sample, or to calibrate the linearity of an analytical instrument. 16. AAS provides an effective and accurate method for determining minute concentrations of ions. Many pollutants and trace elements exist in such small concentrations that it would be impossible to measure their concentration with any degree of accuracy using gravimetric analysis. 17. 5.7 ppm 18. -­‐1 From the graph, the concentration of arsenic before the treatment was about 121 µg L (the same as 121 ppb). After the -­‐1
treatment it has dropped to about 55 µg L . This means that 55% of the original arsenic was removed (121-­‐55/121 x 100). Therefore, the treatment is effective in removing 55% of the arsenic from the water. Page 2