DP Chemistry (last assessment 2024)

Question 20N.3.hl.TZ0.5a

Date	November 2020	Marks available	[Maximum mark: 2]	Reference code	20N.3.hl.TZ0.5a
Level	hl	Paper	3	Time zone	TZ0
Command term	Calculate	Question number	a	Adapted from	N/A

[Maximum mark: 2]

20N.3.hl.TZ0.5a

(a)

Precipitation is one method used to treat waste water.

Phosphates, ${PO}_{4}^{3 -}$ , in waste water can be removed by precipitation with magnesium ions. $K_{s p}$ of magnesium phosphate is $1.04 \times 10^{- 24}$ .

$3 {Mg}^{2 +} (aq) + 2 {PO}_{4}^{3 -} (aq) \to {Mg}_{3} ({PO}_{4})_{2} (s)$

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

[2]

Markscheme

$[{PO}_{4}^{3 -}] = \sqrt{\frac{K_{s p}}{{[{Mg}^{2 +}]}^{3}}}$ ✔

$« [{PO}_{4}^{3 -}] = « \sqrt{\frac{1.04 \times 10^{- 24}}{0 . 0100^{3}}} = » 1.02 \times 10^{- 9} « mol {dm}^{- 3} »$ ✔

Accept “ $K s p = {[M g^{2 +}]}^{3} {[P {O_{4}}^{3 -}]}^{2}$ ” 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.

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

Options » A: Materials

Question 20N.3.hl.TZ0.5a

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