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Date November 2017 Marks available 3 Reference code 17N.3.hl.TZ0.9
Level HL Paper 3 Time zone TZ0
Command term Determine Question number 9 Adapted from N/A

Question

Heavy metal ions are an important environmental concern.

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

[1]
a.

The solubility product, Ksp , 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.

[3]
b.

Markscheme

adsorption

OR

chelation

OR

ion exchange

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

a.

Calculation:

Ksp = [Cd2+] x [S2–] ✔

[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»”.

b.

Examiners report

[N/A]
a.
[N/A]
b.

Syllabus sections

Options » A: Materials » A.10 Environmental impact—heavy metals (HL only)
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