Question 23M.3.HL.TZ2.5
| Date | May 2023 | Marks available | [Maximum mark: 8] | Reference code | 23M.3.HL.TZ2.5 |
| Level | HL | Paper | 3 | Time zone | TZ2 |
| Command term | Determine, Draw, Explain, Identify, State | Question number | 5 | Adapted from | N/A |
Lead can be removed from waste water by precipitation, for example as lead (II) sulfide, PbS, or lead (II) hydroxide, Pb(OH)2.
The unit cell of lead (II) sulfide is shown:
State the coordination number of the lead (II) ion.
[1]
6 ✓
There are 14 sulfide ions shown in the diagram. Explain why the unit cell is only considered to contain 4 sulfide ions.
[2]
6 on faces «shared between two unit cells» so 6x½ = 3 ✓
8 on corners «shared between eight unit cells» so 8x⅛ = 1 ✓
Identify one technique that could be used to determine the structure of lead (II) sulfide.
[1]
X-ray crystallography/diffraction ✓
Accept “electron/neutron diffraction”.
Lead hydroxide is best precipitated at pH 9. Determine the concentration of lead (II) ions present in an aqueous solution of a lead (II) salt at pH 9. Use section 32 of the data booklet.
[3]
[H+] = 10−9 − «mol dm−3» ✓
[OH−] «=» = 10−5 «mol dm−3» ✓
[Pb2+] «=» = 1.43 × 10−10«mol dm−3» ✓
Award [3] for correct final answer.
Accept alternative ways of carrying out the calculation.
An alternative method of removing lead (II) ions from aqueous solutions is chelation.
A possible ligand for this is the anion of iminodiacetic acid.
Draw circles around the atoms on the following ligand, that would bond to the lead (II) ion when it forms a complex ion.
[1]
Accept circles around the carbonyl oxygens instead of those with the negative charges OR circles enclosing both carbonyl oxygens.
Do not award the mark if more than 3 circles are drawn.