| Date | November 2020 | Marks available | 2 | Reference code | 20N.3.hl.TZ0.5 |
| Level | HL | Paper | 3 | Time zone | TZ0 |
| Command term | Calculate | Question number | 5 | Adapted from | N/A |
Question
Precipitation is one method used to treat waste water.
Phosphates, , in waste water can be removed by precipitation with magnesium ions. of magnesium phosphate is .
Calculate the maximum solubility of phosphate ions in a solution containing magnesium ions.
Precipitation is one method used to treat waste water.
Zinc, cadmium, nickel, and lead are metal ions which can be removed by precipitation. Explain why waste water is adjusted to a pH of 9−10 to remove these ions by referring to section 32 of the data booklet.
Markscheme
✔
✔
Accept “” for M1.
Award [2] for correct final answer.
Any two of:
precipitation occurs with a base/carbonate//hydroxide/ ✔
is high enough to cause metal hydroxide precipitation at that ✔
these ions are slightly acidic/more soluble in acidic conditions ✔
only small amounts of carbonate/hydroxides/anion needed at that ✔
solubility products of the hydroxides are very small ✔
Do not accept “hydroxyl” for “hydroxide”.
Examiners report
The Ksp problem was very well answered by the stronger candidates, who had no difficulty working out the maximum solubility of phosphate ions in solution as 1.02 x 10-9 mol dm-3.
This question although somewhat challenging was well answered by the stronger candidates.
Syllabus sections
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18M.3.hl.TZ2.4b.ii:
Vanadium(IV) ions can create free radicals by a Fenton reaction.
Deduce the equation for the reaction of V4+ with hydrogen peroxide.
- 16N.3.hl.TZ0.10a: Compare and contrast the Fenton and Haber–Weiss reaction mechanisms.
- 18N.3.hl.TZ0.3d.i: State one feature of a chelating agent.
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18M.3.hl.TZ2.4b.i:
Vanadium and other transition metals can interfere with cell metabolism.
State and explain one process, other than by creating free radicals, by which transition metals interfere with cell metabolism.
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17N.3.hl.TZ0.9b:
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.
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19M.3.hl.TZ2.8c:
State a method, other than precipitation, of removing heavy metal ions from solution.
- 17N.3.hl.TZ0.9a: State the name of one method, other than precipitation, of removing heavy metal ions from...
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17M.3.hl.TZ2.4a:
Identify the other product formed.
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17M.3.hl.TZ2.5b.iii:
Nickel(II) ions are least soluble at pH 10.5. Calculate the molar solubility of nickel(II) hydroxide at this pH. KspNi(OH)2 = 5.48 × 10–16.
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19M.3.hl.TZ1.7a:
Explain how entropy affects this equilibrium.
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17M.3.hl.TZ1.10c:
Identify a ligand that could be used to chelate antimony(III) ions in solution.
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17M.3.hl.TZ2.4b:
Explain why EDTA, a chelating agent, is more effective in removing heavy metal ions from solution than monodentate ligands.
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18N.3.hl.TZ0.3d.ii:
An aqueous lead(II) ion reacts with three ethane-1,2-diamine molecules to form an octahedral chelate ion.
Outline why the chelate ion is more stable than the reactants.
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19M.3.hl.TZ1.7b:
State the number of coordinate covalent bonds EDTA forms with Ni2+.
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16N.3.hl.TZ0.10b:
Adsorption and chelation are two methods of removing heavy metal ion pollution from the environment.
(i) Describe the process of adsorption.
(ii) Deduce the structure of the complex ion formed by the reaction of three H2N−CH2−CH2−NH2 chelating molecules with a mercury(II) ion.
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17M.3.hl.TZ1.10b:
Hydrogen sulfide could be used to remove antimony(III) ions from a solution.
Determine the concentration of antimony(III) ions that would be required to precipitate antimony(III) sulfide in a solution saturated with hydrogen sulfide.
[S2−] in water saturated with hydrogen sulfide = 1.0 × 10−14 mol dm−3
Ksp (Sb2S3) = 1.6 × 10−93
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18M.3.hl.TZ1.5e:
The concentration of aluminium in drinking water can be reduced by precipitating aluminium hydroxide. Calculate the maximum concentration of aluminium ions in water of pH 7 at 298 K. Solubility product of aluminium hydroxide = 3.3 × 10−34 at 298 K.
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19M.3.hl.TZ2.8b:
Determine the maximum concentration of lead(II) ions at 298 K in a solution in which the concentration of carbonate ions is maintained at 1.10 × 10−4 mol dm−3. Use section 32 of the data booklet.
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19M.3.hl.TZ2.8a:
Outline why heavy metals are toxic.
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19N.3.hl.TZ0.8:
1.40 × 10−3 g of NaOH (s) are dissolved in 250.0 cm3 of 1.00 × 10−11 mol dm−3 Pb(OH)2 (aq) solution.
Determine the change in lead ion concentration in the solution, using section 32 of the data booklet.
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18N.3.hl.TZ0.3b.ii:
Lead ions are toxic and can be precipitated using hydroxide ions.
Pb2+ (aq) + 2OH‒ (aq) Pb(OH)2 (s)
Sufficient sodium hydroxide solid is added to the antacid sample to produce a 1.0 × 10‒2 mol dm‒3 hydroxide ion solution at 298 K.
Deduce if a precipitate will be formed, using section 32 of the data booklet.
If you did not calculate the concentration of lead ions in (b)(i), use the value of 2.4 × 10−4 mol dm‒3, but this is not the correct value.
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20N.3.hl.TZ0.5b:
Precipitation is one method used to treat waste water.
Zinc, cadmium, nickel, and lead are metal ions which can be removed by precipitation. Explain why waste water is adjusted to a pH of 9−10 to remove these ions by referring to section 32 of the data booklet.
