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Question 18N.3.hl.TZ0.3b.ii

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Date November 2018 Marks available [Maximum mark: 2] Reference code 18N.3.hl.TZ0.3b.ii
Level hl Paper 3 Time zone TZ0
Command term Deduce Question number b.ii Adapted from N/A
b.ii.
[Maximum mark: 2]
18N.3.hl.TZ0.3b.ii

The presence of very small amounts of lead in calcium-based antacids can be determined using inductively coupled plasma-mass spectroscopy (ICP-MS).

An unknown antacid sample has a lead ion concentration of 0.50 μg dm‒3.

(b.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.

[2]

Markscheme

«Ksp = 1.43 × 10–20»

 

ALTERNATIVE 1:

«Q = [Pb2+] [OH–]2 = 2.4 × 10–9 × (1.0 × 10–2)2» = 2.4 × 10–13 ✔

 

Q > Ksp AND precipitate will form

OR

2.4 × 10–13 > 1.43 × 10–20 AND precipitate will form ✔

 

ALTERNATIVE 2:

critical [Pb2+] for hydroxide solution « = K s p [ O H − ] 2 = 1.43 × 10 − 20 ( 1.0 × 10 − 2 ) 2 » = 1.4 × 10–16 ✔

 

initial concentration > critical concentration AND precipitate will form

OR

2.4 × 10–9 > 1.4 × 10–16 AND precipitate will form ✔

 

If value given is used:

ALTERNATIVE 3:

«Q = [Pb2+] [OH–]2 = 2.4 × 10–4 × (1.0 × 10–2)2» = 2.4 × 10–8 ✔

 

Q > Ksp AND precipitate will form

OR

2.4 × 10–8 > 1.43 × 10–20 AND precipitate will form ✔

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