Question 18N.3.sl.TZ0.f.i
| Date | November 2018 | Marks available | [Maximum mark: 2] | Reference code | 18N.3.sl.TZ0.f.i |
| Level | sl | Paper | 3 | Time zone | TZ0 |
| Command term | Deduce | Question number | f.i | Adapted from | N/A |
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 by UV-vis spectrometry.
A sample of brass is dissolved in concentrated nitric acid and then made up to 250.0 cm3 with water 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.
Titration is another method for analysing the solution obtained from adding brass to nitric acid.
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.
[2]
2I− (aq) + 2Cu2+ (aq) + 2S2O32− (aq) → 2CuI (s) + S4O62− (aq)
correct reactants and products ✔
balanced equation ✔
M2 can only be awarded if M1 is correct.
