| Date | November 2009 | Marks available | 2 | Reference code | 09N.3.hl.TZ0.E5 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate | Question number | E5 | Adapted from | N/A |
Question
Compounds of heavy metals are one type of toxic substance found in water. Lead(II) ions, \({\text{P}}{{\text{b}}^{2 + }}\), can be removed by bubbling hydrogen sulfide, \({{\text{H}}_{\text{2}}}{\text{S}}\), through polluted water. The solubility product of lead sulfide is \({\text{1.25}} \times {\text{1}}{{\text{0}}^{ - 28}}{\text{ mo}}{{\text{l}}^{\text{2}}}{\text{d}}{{\text{m}}^{ - 6}}\) at 25 °C.
Calculate the concentration of \({\text{P}}{{\text{b}}^{2 + }}\) ions in a saturated solution of lead sulfide.
Explain how the addition of hydrogen sulfide decreases the concentration of \({\text{P}}{{\text{b}}^{2 + }}\) ions in a saturated solution.
Markscheme
\({K_{{\text{sp}}}} = {\text{[P}}{{\text{b}}^{2 + }}{\text{][}}{{\text{S}}^{2 - }}{\text{]}}\);
\({\text{[P}}{{\text{b}}^{2 + }}{\text{]}} = 1.12 \times {10^{ - 14}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}\);
Award [2] for final correct answer.
increase in \({\text{[}}{{\text{S}}^{2 - }}{\text{]}}\) (from hydrogen sulfide);
causes PbS to precipitate / shifts equilibrium direction to form PbS;
Allow “common ion effect” as alternative to either of the above.
Examiners report
The better candidates were able to solve the \({K_{sp}}\) problem in (a).
Although the better candidates were able to solve the \({K_{sp}}\) problem in (a), most did not know the subsequent answer to (b).
