| Date | May 2013 | Marks available | 2 | Reference code | 13M.3.hl.TZ2.E4 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Explain | Question number | E4 | Adapted from | N/A |
Question
In order to make waste water acceptable for drinking, it is treated in a series of steps to remove hazardous substances.
Tertiary treatment removes phosphates, nitrates and heavy metal ions from water.
The solubility product constant, \({K_{{\text{sp}}}}\), of cadmium(II) sulfide, CdS, is \({\text{8.00}} \times {\text{1}}{{\text{0}}^{ - 28}}\) at 298 K. Determine the concentration of cadmium(II) ions, \({\text{C}}{{\text{d}}^{2 + }}{\text{(aq)}}\), in a saturated solution of cadmium(II) sulfide.
Explain how the addition of hydrogen sulfide gas can decrease the concentration of cadmium(II) ions in a saturated solution.
Markscheme
\({\text{(}}{K_{{\text{sp}}}} = {\text{) [C}}{{\text{d}}^{2 + }}{\text{(aq)][}}{{\text{S}}^{2 - }}{\text{(aq)]}}\);
\({\text{[C}}{{\text{d}}^{2 + }}{\text{(aq)]}} = {\text{2.83}} \times {\text{1}}{{\text{0}}^{ - 14}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);
Award [2] for correct final answer.
Ignore state symbols.
increase in \(\left[ {{{\text{S}}^{2 - }}{\text{(aq)}}} \right]\);
causes CdS precipitate to form/equilibrium shifts to form more CdS/common ion effect / OWTTE;
Examiners report
The question on the solubility of cadmium was done reasonably well but many erroneously produced hydrogen gas instead of hydrogen ions in their equation and hence lost both marks.
\({K_{{\text{sp}}}}\) was often calculated correctly in b) i) although the arguments around the common ion effect in b) ii) showed a poor understanding.
The question on the solubility of cadmium was done reasonably well but many erroneously produced hydrogen gas instead of hydrogen ions in their equation and hence lost both marks.
\({K_{{\text{sp}}}}\) was often calculated correctly in b) i) although the arguments around the common ion effect in b) ii) showed a poor understanding.
