Question 18N.3.sl.TZ0.1f.ii
| Date | November 2018 | Marks available | [Maximum mark: 1] | Reference code | 18N.3.sl.TZ0.1f.ii |
| Level | sl | Paper | 3 | Time zone | TZ0 |
| Command term | Suggest | Question number | f.ii | 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.
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.
[1]
precipitate/copper(I) iodide/CuI makes colour change difficult to see
OR
release of I2/iodine from starch-I2 complex is slow so titration must be done slowly ✔
