State the expression for the solubility product constant, ${K_{{\text{sp}}}}$, of lead sulfate.

Deduce why lead sulfate will not precipitate out of the water sample at these concentrations.

Some magnesium sulfate is added to the water sample. Determine the increase in sulfate ion concentration needed for lead sulfate to precipitate.

$({K_{{\text{sp}}}} = ){\text{ }}[{\text{P}}{{\text{b}}^{2 + }}][{\text{SO}}_4^{2 - }]$;

$[{\text{P}}{{\text{b}}^{2 + }}][{\text{SO}}_4^{2 - }]/2.32 \times {10^{ - 6}} \times 4.15 \times {10^{ - 3}}/9.63 \times {10^{ - 9}}$ is less than ${K_{{\text{sp}}}}/1.80 \times {10^{ - 8}}$;

$(1.80 \times {10^{ - 8}} = 2.32 \times {10^{ - 6}} \times [{\text{SO}}_4^{2 - }]),{\text{ }}[{\text{SO}}_4^{2 - }] = 7.76 \times {10^{ - 3}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ - 3}}{\text{)}}$;

increase in $[{\text{SO}}_4^{2 - }]{\text{ }}( = 7.76 \times {10^{ - 3}} - 4.15 \times {10^{ - 3}}) = 3.61 \times {10^{ - 3}}{\text{ }}({\text{mol}}\,{\text{d}}{{\text{m}}^{ - 3}})$;

Award [2] for correct final answer.

It was very surprising in (a) at the number of candidates who simply could not write a correct ${K_{{\text{sp}}}}$ expression for lead sulfate.

Only the stronger candidates got (b) and subsequently (c) correct.

Only the stronger candidates got (b) and subsequently (c) correct.