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.

${K_{{\text{sp}}}} = {\text{[P}}{{\text{b}}^{2 + }}{\text{][}}{{\text{S}}^{2 - }}{\text{]}}$;

${\text{[P}}{{\text{b}}^{2 + }}{\text{]}} = 1.12 \times {10^{ - 14}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}$;

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.

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).