DP Chemistry (last assessment 2024)

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Question 20N.3.hl.TZ0.5

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Date November 2020 Marks available [Maximum mark: 4] Reference code 20N.3.hl.TZ0.5
Level hl Paper 3 Time zone TZ0
Command term Calculate, Explain Question number 5 Adapted from N/A
5.
[Maximum mark: 4]
20N.3.hl.TZ0.5
(a)

Precipitation is one method used to treat waste water.

Phosphates, PO43-, in waste water can be removed by precipitation with magnesium ions. Ksp of magnesium phosphate is 1.04×10-24.

3Mg2+(aq)+2PO43-(aq)→Mg3(PO4)2(s)

Calculate the maximum solubility of phosphate ions in a solution containing 0.0100 mol dm-3 magnesium ions.

[2]

Markscheme

PO43-=KspMg2+3 ✔

«PO43−=«1.04×10-240.01003=»1.02×10−9«mol dm−3» ✔


Accept “Ksp=Mg2+3PO43-2” for M1.
Award [2] for correct final answer.

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.

(b)

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.

[2]

Markscheme

Any two of:
precipitation occurs with a base/carbonate/CO32−/hydroxide/OH− ✔

OH− is high enough to cause metal hydroxide precipitation at that pH ✔

these ions are slightly acidic/more soluble in acidic conditions ✔

only small amounts of carbonate/hydroxides/anion needed at that pH ✔

solubility products of the hydroxides are very small ✔

Do not accept “hydroxyl” for “hydroxide”.

Examiners report

This question although somewhat challenging was well answered by the stronger candidates.

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