2018年11月IB化学SL真题下载-Paper3
1.Alloys containing at least 60 % copper reduce the presence of bacteria on their surface.The percentage of copper in brass, an alloy of copper and zinc, can be determined byUV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to 250.0 cm3 withwater before analysis.
Cu (s) + 4HNO3 (aq) → Cu(NO3)2 (aq) + 2NO2 (g) + 2H2O (l)
3Zn (s) + 8HNO3 (aq) → 3Zn(NO3)2 (aq) + 2NO (g) + 4H2O (l)
The concentration of copper(II) ions in the resulting solution is then determined from a calibration curve, which is plotted by measuring the light absorbance of standard solutions.
(a) Outline why the initial reaction should be carried out under a fume hood.
(b) Deduce the equation for the relationship between absorbance and concentration.
Slope (gradient):
Equation:
(c) Outline how a solution of 0.0100 mol dm−3 is obtained from a standard 1.000 mol dm−3 copper(II) sulfate solution, including two essential pieces of glassware you would need.
(d) (i) The original piece of brass weighed 0.200 g. The absorbance was 0.32.Calculate, showing your working, the percentage of copper by mass in the brass.
(ii) Deduce the appropriate number of significant figures for your answer in (d)(i).
(e) (i) Comment on the suitability of using brass of this composition for door handles in hospitals. If you did not obtain an answer to (d)(i), use 70 % but this is not the correct answer.
(ii) Suggest another property of brass that makes it suitable for door handles.
(f) Titration is another method for analysing the solution obtained from adding brass to nitric acid.
(i) Copper(II) ions are reduced to copper(I) iodide by the addition of potassium iodide solution, releasing iodine that can be titrated with sodium thiosulfate solution, Na2S2O3 (aq). Copper(I) iodide is a white solid.
4I− (aq) + 2Cu2+ (aq) → 2CuI (s) + I2 (aq)
I2 (aq) + 2S2O32− (aq) → 2I− (aq) + S4O62− (aq)
Deduce the overall equation for the two reactions by combining the two equations.
(ii) Suggest why the end point of the titration is difficult to determine, even with the addition of starch to turn the remaining free iodine black.
2018年11月IB化学SL真题余下省略！
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