State the name of one method, other than precipitation, of removing heavy metal ions from solution in water.

The solubility product, K_sp , of cadmium sulfide, CdS, is 8.0 × 10^–27. Determine the concentration of cadmium ions in 1.0 dm³ of a saturated solution of cadmium sulfide to which 0.10 mol of solid sodium sulfide has been added, stating any assumption you make.

adsorption

OR

chelation

OR

ion exchange

Accept other valid methods such as “phytoremediation” OR “Fenton reaction” OR “electrolysis”.

Calculation:

K_sp = [Cd²⁺] x [S^2–] ✔

[Cd²⁺] = 8.0 x 10^–26 «mol dm^–3» ✔

Assumption:

volume of solution remains 1.0 dm³

OR

concentration of sulfide ions in original solution is negligible

OR

hydrolysis of sulfide ions is negligible

Award [2] for correct numerical answer of [Cd²⁺] for M1 and M2.

Accept “0.10 + x ∼ 0.10 «mol dm^–3»”.