Question 18N.3.sl.TZ0.1d.i
| Date | November 2018 | Marks available | [Maximum mark: 3] | Reference code | 18N.3.sl.TZ0.1d.i |
| Level | sl | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate | Question number | d.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.
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.
[3]
concentration of copper = 0.0080 «mol dm–3» ✔
mass of copper in 250.0 cm3 = «0.0080 mol dm–3 × 0.2500 dm3 × 63.55 g mol–1 =» 0.127 «g»
OR
mass of brass in 1 dm3 = «4 × 0.200 g =» 0.800 g AND [Cu2+] = «0.0080 mol dm–3 × 63.55 g mol–1 =» 0.5084 g dm–3 ✔
«% copper in this sample of brass » 64 «%»
OR
«% copper in this sample of brass » 64 «%» ✔
Accept any value in range 0.0075–0.0085 «mol dm–3» for M1.
Accept annotation on graph for M1.
Award [3] for correct final answer.
Accept “65 «%»”.
