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Date November 2018 Marks available 1 Reference code 18N.3.sl.TZ0.3
Level SL Paper 3 Time zone TZ0
Command term State Question number 3 Adapted from N/A

Question

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

State the type of particle present in the plasma formed.

[1]
a.

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

Calculate the concentration of lead ions in the sample in mol dm‒3.

[2]
b.

Electrolysis is used to obtain lead from Pb2+ (aq) solution.

Determine the time, in hours, required to produce 0.0500 mol lead using a current (I) of 1.34 A. Use section 2 of the data booklet and the equation, charge (Q) = current (I) × time (t, in seconds).

[2]
c.

Markscheme

positive ions/cations/Pb2+

OR

free electrons ✔

 

Accept “ions” OR “charged species/particle”.

a.

[Pb2+] = 0.50 × 10‒6/5.0 × 10–7 «g dm–3» ✔

[Pb2+] « = 0.50 × 10 6 g d m 3 207.20 g mo l 1 » = 2.4 × 10‒9 «mol dm‒3» ✔

 

Award [2] for correct final answer.

b.

«Faraday’s constant, F = 9.65 × 104 C mol‒1 and 1 A = 1 C s–1»
Q «= 0.0500 mol × 2 × 96500 C mol‒1» = 9650 «C» ✔

t « = Q I = 9650 C 1.34 C s 1 7200 s  so  7200 s 60 × 60 s h 1 » = 2.00 «hours» ✔

 

Award [2] for correct final answer.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Options » A: Materials » A.2 Metals and inductively coupled plasma (ICP) spectroscopy
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