DP Chemistry (last assessment 2024)

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Question 20N.3.hl.TZ0.b

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Date November 2020 Marks available [Maximum mark: 2] Reference code 20N.3.hl.TZ0.b
Level hl Paper 3 Time zone TZ0
Command term Explain Question number b Adapted from N/A
b.
[Maximum mark: 2]
20N.3.hl.TZ0.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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