XXX_1

PRACTICALEXAM

Making science together!

2019-07-24

XXX_1
General instructions
This practical booklet contains 27 pages.
Before the start of the practical exam, the Read command is given. You will have 15
minutes to read the exam booklet. You may only read during this time; do not write nor use thecalculator.

You may begin working as soon as the Start command is given. You will then have 5 hours
to complete the exam.
You may work on the tasks in any order, but starting with problem P1 is advised.
All results and answers must be clearly written in pen in their respective designed areas
on the exam papers. Answers written outside the answer boxes will not be graded.

If you need scratch paper, use the backside of the exam sheets. Remember that nothingoutside the designed areas will be graded.
The official English version of the exam booklet is available upon request and serves forclarification only.

If you need to leave the laboratory (to use the restroom or have a drink or snack), raise theappropriate card. A lab assistant will come to accompany you.

Shelves above the benches are not to be used during the task for the purpose of equality.
You must follow the safety rules given in the IChO regulations. If you break the safety
rules, you will receive only one warning from the lab assistant. Any safety rule violation after the
first warning will result in your dismissal from the laboratory and the nullification of your practicalexamination.
Chemicals and labware, unless otherwise noticed, will be refilled or replaced without
penalty only for the first incident. Each further incident will result in the deduction of 1 point fromyour 40 practical exam points.

The lab supervisor will announce a 30 minutes warning before the Stop command.
You must stop your work immediately when the Stop command is announced. Failure tostop working or writing by one minute or longer will lead to nullification of your practical exam.

After the Stop command has been given, the lab supervisor will come to sign your answer
sheet.
After both the supervisor and you sign, place this exam booklet in the envelope and submit itfor grading together with your product and thin-layer chromatography (TLC) plates.
Lab rules and safety
You must wear a lab coat and keep it buttoned up. Footwear must completely cover the footand the heel.

Always wear safety glasses or prescription glasses when working in the lab. Do not wear
contact lenses.

Do not eat or drink in the lab. Chewing gums are not allowed.
Work only in the designated area. Keep your work area and the common work areas tidy.
No unauthorized experiments are allowed. No modification of the experiments is allowed.

Do not pipette with your mouth. Always use a pipette filler bulb.

Clean up spills and broken glassware immediately from both the bench and the floor.
All waste must be properly discarded to prevent contamination or injury. Water solutions are
eligible for sink disposal. Organic waste must be disposed of in the marked capped container.

51st IChO – Practical Exam2

XXX_1
Physical constants and equations
In these tasks, we assume the activities of all aqueous species to be well approximated by theirrespective concentration in mol L−1. To further simplify formulae and expressions, the standard
concentration c° = 1 mol L−1 is omitted.
Avogadro's constant:
Universal gas constant:Standard pressure:
Atmospheric pressure:
Zero of the Celsius scale:
Faraday constant:
Watt:
Kilowatt hour:
Planck constant:
Speed of light in vacuum:
Elementary charge:
Electrical power:
Power efficiency:
Planck-Einstein relation:
Ideal gas equation:
Gibbs free energy:

Reaction quotient 𝑄 for a reactiona A(aq) + b B(aq) = c C(aq) + d D(aq):Henderson−Hasselbalch equation:
Nernst–Peterson equation:
where Q is the reaction quotient of the
reduction half-reaction
Beer–Lambert law:
Rate laws in integrated form:- Zero order:- First order:- Second order:
Half-life for a first order process:Number average molar mass Mn:
Mass average molar mass Mw:
Polydispersity index Ip:

NA = 6.022∙1023 mol−1
R = 8.314 J mol−1 K−1
p° = 1 bar = 105 Pa
Patm = 1 atm = 1.013 bar = 1.013∙105 Pa
273.15 K
F = 9.649∙104 C mol−1
1 W = 1 J s−1
1 kWh = 3.6∙106 J
h = 6.626∙10−34 J s
c = 2.998∙108 m s−1
e = 1.6022∙10−19 C
P = ΔE×Iη = Pobtained/Papplied
E = hc/λpV = nRT
G = H – TS
ΔrG° = −RT lnK°ΔrG° = −n F Ecell°
ΔrG = ΔrG° + RT lnQ𝑄=[C]c [D]d[A]a [B]b

[A− ]
[AH]
RT
o
E = E − ln𝑄

pH = pKa + log

zF
RTat T = 298 K, F ln10 ≈ 0.059 V

A = εlc[A] = [A]0 – ktln[A] = ln[A]0 − kt
1/[A] = 1/[A]0 + kt
t1/2 = ln2/k∑i 𝑁i 𝑀i𝑀n =∑i 𝑁i
∑i 𝑁i 𝑀2i
𝑀w =∑i 𝑁i 𝑀i
Mw
Ip =
Mn

Note
The unit of molar concentration is either “M” or “mol L‒1”:
1 M = 1 mol L‒1
1 mM = 10‒3 mol L‒1
1 µM = 10‒6 mol L‒1

51st IChO – Practical Exam3

XXX_1
Periodic table

1

18

1

H

2

He

2

13

14

15

16

17

3

4

5

6

7

8

9

10

Li

Be

B

C

N

O

F

Ne

6.94

9.01

10.81

12.01

14.01

16.00

19.00

20.18

11

12

13

14

15

16

17

18

1.008

4.003

Na Mg
22.99

24.31

19

20

3
21

4
22

5
23

6
24

7
25

8
26

9
27

10
28

11
29

12
30

Al

Si

P

S

Cl

Ar

26.98

28.09

30.97

32.06

35.45

39.95

31

32

33

34

35

36

K

Ca Sc

Ti

V

Cr

Mn Fe

Co

Ni

Cu

Zn

Ga Ge

As

Se

Br

Kr

39.10

40.08

44.96

47.87

50.94

52.00

54.94

55.85

58.93

58.69

63.55

65.38

69.72

72.63

74.92

78.97

79.90

83.80

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

53

54

Rb

Sr

Y

Zr

Nb Mo

85.47

87.62

88.91

91.22

92.91

55

56

72

73

Cs

Ba 57-71

Hf

Ta

132.9

137.3

178.5

180.9

183.8

186.2

87

88

104

105

106

107

Fr

Ra 89103

Rf

Db

Sg

-

-

-

-

-

Ru Rh

Pd

Ag

Cd

In

Sn

Sb

Te

I

Xe

-

101.1

102.9

106.4

107.9

112.4

114.8

118.7

121.8

127.6

126.9

131.3

74

75

76

77

78

79

80

81

82

83

84

85

86

W

Re Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

190.2

192.2

195.1

197.0

200.6

204.4

207.2

209.0

-

-

-

108

109

110

111

112

113

114

115

116

117

118

Bh

Hs

Mt

Ds

Rg Cn Nh

Fl

Mc

Lv

Ts

Og

-

-

-

-

-

-

-

-

-

-

61

62

63

69

95.95

Tc

-

-

57

58

59

60

64

65

66

67

68

70

71

La

Ce

Pr

Nd Pm Sm Eu Gd

Tb

Dy

Ho

Er

Tm Yb

Lu

138.9

140.1

140.9

144.2

-

150.4

152.0

157.3

158.9

162.5

164.9

167.3

168.9

173.0

175.0

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

Ac

Th

Pa

U

Np

Pu Am Cm Bk

Cf

Es Fm Md No

Lr

-

232.0

231.0

238.0

-

51st IChO – Practical Exam

-

-

-

-

-

-

-

-

-

-

4

XXX_1
Definition of GHS statements
The GHS hazard statements (H-phrases) associated with the materials used are indicated in the
problems. Their meanings are as follows.
Physical hazards
H225 Highly flammable liquid and vapor.
H226 Flammable liquid and vapor.
H228 Flammable solid.
H271 May cause fire or explosion; strong oxidizer.
H272 May intensify fire; oxidizer.
H290 May be corrosive to metals.
Health hazards
H301 Toxic if swallowed.
H302 Harmful if swallowed.
H304 May be fatal if swallowed and enters airways.
H311 Toxic in contact with skin.
H312 Harmful in contact with skin.
H314 Causes severe skin burns and eye damage.
H315 Causes skin irritation.
H317 May cause an allergic skin reaction.
H318 Causes serious eye damage.
H319 Causes serious eye irritation.
H331 Toxic if inhaled.
H332 Harmful if inhaled.
H333 May be harmful if inhaled.
H334 May cause allergy or asthma symptoms or breathing difficulties if inhaled.
H335 May cause respiratory irritation.
H336 May cause drowsiness or dizziness.
H351 Suspected of causing cancer.
H361 Suspected of damaging fertility or the unborn child.
H371 May cause damage to organs.
H372 Causes damage to organs through prolonged or repeated exposure.
H373 May cause damage to organs through prolonged or repeated exposure.
Environmental hazards
H400 Very toxic to aquatic life.
H402 Harmful to aquatic life.
H410 Very toxic to aquatic life with long-lasting effects.
H411 Toxic to aquatic life with long-lasting effects.
H412 Harmful to aquatic life with long-lasting effects.

51st IChO – Practical Exam5

XXX_1
Chemicals
For all problems
Chemicals
Deionized water in:
Wash bottle (bench)
Plastic bottle (bench)
Plastic canister (hood)
Ethanol, in a wash bottle
Sample of white wine, 300 mL inamber plastic bottle
For problem P1
Chemicals
4-nitrobenzaldehyde, 1.51 g inamber glass vial

Labeled as

GHS hazard statements

Deionized Water

Not hazardous

Ethanol

H225, H319

Wine sample

H225, H319

Labeled as

GHS hazard statements

4-nitrobenzaldehyde

H317, H319

Eluent A, 20 mL in glass vial

Eluent A

Eluent B, 20 mL in glass vial

Eluent B

Oxone® (potassium
peroxomonosulfate salt), 7.87 g inplastic bottle
Sample of 4-nitrobenzaldehyde for
TLC
For problem P2
Chemicals
1 M potassium thiocyanate
solution, 20 mL in plastic bottle
0.00200 M potassium thiocyanate
solution, 60 mL in plastic bottle
1 M perchloric acid solution, 10
mL in plastic bottle
0.00200 M iron(III) solution, 80
mL in plastic bottle
0.000200 M iron(III) solution, 80
mL in plastic bottle
0.3% hydrogen peroxide solution, 3
mL in amber glass bottle

51st IChO – Practical Exam

H225, H290, H304, H314,
H319, H336, H410
H225, H290, H304, H314,
H319, H336, H410

Oxone®

H314

TLC standard

H317, H319

Labeled as

GHS hazard statements

KSCN 1 M

H302+H312+H332, H412

KSCN 0.00200 M

Not hazardous

HClO4

H290, H315, H319

Fe(III) 0.00200 M

Not hazardous

Fe(III) 0.000200 M

Not hazardous

H2O2

Not hazardous6

XXX_1
For problem P3
Chemicals
0.01 M iodine solution, 200 mL inbrown plastic bottle
0.03 M sodium thiosulfate solution,
200 mL in plastic bottle
1 M NaOH solution, 55 mL in plastic
bottle2.5 M sulfuric acid solution, 80 mL inplastic bottle
0.5 M potassium iodide solution,
25 mL in plastic bottle
Potassium iodate, ca 100 mg (exact
mass written on the label), in glass vial
Starch solution, 25 mL in plastic bottle

51st IChO – Practical ExamLabeled as

GHS hazard statements

I2

H372

Na2S2O3

Not hazardous

NaOH

H290, H314

H2SO4

H290, H315, H319

KI

H372

KIO3

H272, H315, H319, H335

StarchNot hazardous7

XXX_1Equipment
For all problems
Personal equipment
Pipette filler bulb
Safety goggles
1 L plastic bottle for organic waste, labeled “Organic
waste”
Paper towels
Precision wipers
Spatula (large)
Spatula (small)
Stopwatch
PencilEraserBlack penFelt-tip pen for glassware
Ruler
Shared equipment
UV lamp for TLC visualization
Colorimeter
GlovesIce bucket
For problem P1
Personal equipment
Laboratory stand with:
Clamp holder with small clamp
Clamp holder with large clamp
Erlenmeyer flask with ground joint, 100 mL
Erlenmeyer flask with ground joint, 50 mL
Reflux condenser
Hotplate stirrer
Crystallizing dishMagnetic stirring bar
Suction flask
Büchner funnel with rubber adapter
Zipped bag with 3 pieces of filter paper
Petri dishTLC elution chamber, labeled “TLC elution chamber”
Zipped bag with 3 TLC plates (with fluorescenceindicator), labeled with Student Code
TLC graduated spotters (in the Petri dish)
Plastic tweezers
Glass rod
Graduated cylinder, 25 mL
Beaker, 150 mL
Plastic powder funnel
Disposable plastic pipette
51st IChO – Practical Exam

Quantity
1
1
1
15 sheets
30 sheets
1
1
1
1
1
1
1
1
Quantity
2 per lab
5 per lab
All sizes (S, M, L, XL) available
upon request to a lab assistant
1 per lab

Quantity
1
2
1
1
1
1
1
1
1
1
1
1
1
1
1
4
1
1
1
2
1
2
8

XXX_1
Amber glass vial, for TLC sample, 1.5 mL, withstopper, labeled C and R
Pre-weighed amber glass vial, 10 mL, with stopper,
labeled with Student CodeMagnetic stirring bar retriever2
1
1

For problem P2
Personal equipment
Volumetric pipette, 10 mL
Graduated pipette, 10 mL
Graduated pipette, 5 mL
Test tube stand
Test tube
Test tube stopper
Colorimeter cuvette, path length 1.0 cm
Beaker, 100 mL
Disposable plastic pipette

Quantity
1
3
3
1
15
7
2
2
15

For problem P3
Personal equipment
Laboratory stand with burette clamp
Burette, 25 mL
Glass transfer funnel
Erlenmeyer flask, 100 mL
Erlenmeyer flask, 250 mL
Beaker, 150 mL
Beaker, 100 mL
Volumetric flask, 100 mL, with stopper
Volumetric pipette, 50 mL
Volumetric pipette, 25 mL
Volumetric pipette, 20 mL
Graduated cylinder, 25 mL
Graduated cylinder, 10 mL
Graduated cylinder, 5 mL
Disposable plastic pipette
Parafilm

Quantity
1
1
1
3
3
1
2
1
1
1
1
1
1
1
3
20 sheets

51st IChO – Practical Exam

9

XXX_1
Problem
P1
13% oftotal

Question

Yield

Purity

TLC

P1.1

P1.2

Total

Points

12

12

8

2

3

37

Score

Problem P1. Greening the oxidation of nitrobenzaldehyde
For the last decades, chemists have tried to replace harmful reagents in oxidation processes in order to reduce
hazardous waste treatment. In this problem, potassium peroxomonosulfate has been chosen as oxidizing
agent, because it only produces non-toxic and non-polluting sulfate salts. It is provided here as Oxone®.
Furthermore, the reaction itself is performed in a mixture of water and ethanol, which are classified as green
solvents.
Your task is to perform the oxidation of 4-nitrobenzaldehyde, to recrystallize the product, to compare
TLC eluents and to check the purity of the product using TLC.
Note: Ethanol waste and eluent must be disposed of in the “Organic waste” bottle.

Procedure
I.

Oxidation of 4-nitrobenzaldehyde1.

Mix 20 mL of water and 5 mL of ethanol.

2.

Insert the magnetic bar in the 100 mL ground-joint Erlenmeyer flask.

3.
Transfer the pre-weighed 1.51 g of 4-nitrobenzaldehyde into the Erlenmeyer flask. Add allof the water/ethanol mixture prepared previously. Clamp the Erlenmeyer flask to the stand. Start
stirring the mixture, then add the pre-weighed 7.87 g of Oxone®.
4.
Attach the reflux condenser by loosening the large clamp and adjusting the ground joints(see Figure 1). Raise your HELP card. A lab assistant will come to turn on the water and set the
hotplate.
5.
Heat the reaction mixture with a gentle reflux (ca 1 drop refluxing per second) for 45
minutes. The mark on the heater corresponds to the necessary power to get a gentle reflux.

large clamp

small clamp

Figure 1. Setup for heating the reaction mixture under reflux

51st IChO – Practical Exam10

XXX_1
6.
Then turn off the heating on the hotplate stirrer. Remove the hot plate and let the reaction
mixture cool down for 10 minutes. Place it afterwards in the crystallizing dish filled with an icewater mixture. Let it stand for another 10 minutes.
7.
Set up a vacuum filtration apparatus (see Figure 2) using a Büchner funnel, a filter paper
and a suction flask, that is secured to the laboratory stand with a small clamp. Raise your HELP
card. A lab assistant will come and show how to connect the suction flask to the vacuum source.

Büchner funnel
Filter paper
Rubber adapter
Small clamp

VacuumFigure 2. Setup for the vacuum filtration

8.

Wet the filter paper with water and ensure that it covers all the holes of the Büchner funnel.

9.
Pour the suspension of the crude product into the Büchner funnel and apply vacuum. Wash
the solid thoroughly with deionized water (at least 4×20 mL).
10.
Let air suck through the precipitate for 5 minutes to pre-dry the product. Disconnect thevacuum source. Use the small spatula to transfer one tip of spatula of the product in the 1.5 mL
amber glass vial, labeled C. Close the vial and save it for part III.
11.

Transfer all of the remaining solid into the 50 mL ground-joint Erlenmeyer flask.

12.
Discard the filtrate in the “Organic waste” bottle and wash both the suction flask and the
Büchner funnel with ethanol and water. Use the “Organic waste” bottle to dispose of ethanol waste.
II.

Recrystallization of the product1.

Mix 9 mL of water and 21 mL of ethanol.

2.
Perform the recrystallization of the crude product contained in the 50 mL ground-joint
Erlenmeyer flask with the appropriate amount of this water/ethanol mixture, using the same setup as
for the reflux heating (see Figure 1). Raise your HELP card. A lab assistant will come to turn on thewater and set the hotplate. Add the solvent by the top of the condenser, if needed.
3.
Once the product has crystallized, use the same procedure as described previously (I.7 toI.10) to collect the solid. Use the small spatula to transfer one tip of spatula of the recrystallized
product in the 1.5 mL amber glass vial, labeled R. Close the vial and save it for part III.

51st IChO – Practical Exam11

XXX_1
4.
Transfer the purified solid in the pre-weighed vial labeled with your Student Code. Closethe vial.
5.
Discard the filtrate in the “Organic waste” bottle and raise your HELP card. A lab assistantwill come to turn off the water of the condenser.
III.

TLC analysis

1.
Prepare the elution chamber. Load the elution chamber with ca 0.5 cm in height of eluent
A. Cover it with a Petri dish. Wait for the eluent to saturate the atmosphere in the elution chamber.
2.
Prepare your samples. You are provided a sample of 4-nitrobenzaldehyde in an amberglass vial labeled TLC standard (referred as S on the TLC). You have also kept a small sample ofyour crude product (vial C) and your recrystallized product (vial R) in two other amber glass vials.
Add ca 1 mL of ethanol in each of the vials in order to dissolve the samples.
3.
Prepare your TLC plate. Use a pencil to draw carefully the start line (1 cm above thebottom of the plate) and mark the positions in order to spot the 3 samples. Label them S (Starting
material), C (Crude product) and R (Recrystallized product), as described in Figure 3. On the top
left of the plate, write your Student Code. On the top right of the plate, write the eluent you use(first Eluent A, then Eluent B). Spot the three samples on the plate, using capillary spotters.

Figure 3. TLC plate preparation

4.
Perform the TLC analysis. Using tweezers, insert the TLC plate into the elution chamberand cover it with the Petri dish. Let the eluent reach approximately 1 cm below the top of the plate.
Using tweezers, remove the plate, mark the eluent front with a pencil and let the plate air-dry.
5.
Visualize the TLC plate. Place the TLC plate under the UV lamp kept on the common
bench. With a pencil, circle all the visible spots.
6.

Discard the eluent into the “Organic waste” bottle.

7.

Repeat steps 1, 3, 4, 5, and 6 with eluent B.

8.

Place your plates in the zipped bag with your Student Code.

51st IChO – Practical Exam

12

XXX_1Results of your TLC analysis (complete the schemes with your results). You may use thesedrawings to make a scheme of your TLC plates that may help you answer the following questions.
The scheme will not be graded.

At the end of the examination, your lab supervisor will pick up the following items:
Glass vial labeled with your Student Code containing your recrystallized product;
TLC plates A and B in zipped bag labeled with your Student Code.
Submitted items
Recrystallized product
TLC plate A
TLC plate B□□□

Signatures
Student

51st IChO – Practical ExamLab Supervisor

13

XXX_1
Questions1.
Propose a structure for the final organic product from the reaction of 4-nitrobenzaldehyde
and Oxone®.

2.

Based on your results on the TLC analysis, answer the following questions.



Which eluent is better to follow the reaction progress?

□A


□B
The crude product (C) contains traces of 4-nitrobenzaldehyde.

□ True□ False

The recrystallized product (R) contains traces of 4-nitrobenzaldehyde.

□ True51st IChO – Practical Exam

□ False14

XXX_1
Problem
P2
14% oftotal

Question

Calibration

Points10

Iron
determination
6

P2.1

P2.2

P2.3

3

4

3

Stoichiometry
determination
9

P2.4

P2.5

Total

3

2

40

Score

Problem P2. The iron age of wine
Iron is an element which can naturally be found in wine. When its concentration exceeds 10 to 15 mg perliter, iron(II) oxidation into iron(III) may lead to quality loss, through the formation of precipitates. It istherefore necessary to assess the iron content of the wine during its production.
Given the very low concentration of iron species, a colored complex of iron(III) with thiocyanate SCN‒ as aligand is used to quantify the iron amount, through spectrophotometric measurements.
Your task is to determine the total iron concentration of the white wine provided, using
spectrophotometry, and to determine the stoichiometry of the thiocyanate – iron(III) complex.

WARNINGIn this task, you are provided with two iron(III) solutions and two potassium thiocyanate
solutions of different concentrations. Be very careful not to confuse them.
Once the solutions are ready for spectrophotometric measurements, record the absorbanceno later than one hour after the addition of thiocyanate.
When you need a colorimeter, raise your HELP card. A lab assistant will give you acolorimeter labeled. You will have the exclusive use of this colorimeter for up to 15 minutes. The
lab assistant will take it back as soon as you have finished or when the 15 minutes are over. If nocolorimeter is available at the precise moment, you will be added to a waiting-list.
Instructions for the colorimeter are presented on the following page.
You can call for the colorimeter only three times for this problem.

51st IChO – Practical Exam

15

XXX_1
Instructions for the use of the colorimeter
Absorbance/transmittance mode

Transmission

Absorbance

Display
Wavelength
selection

Orientation
schemeSelection
wheel

Cuvettetank

OK button


Plug in the colorimeter.

Check that “Absorbance” is highlighted. If not, turn the selection wheel until a dashed line
appears around “Absorbance” and then press the OK button.
Turn the selection wheel until a dashed line appears around the desired wavelength(470 nm). Press the OK button.
Place the cuvette with ca 3 cm in height of the blank solution in the tank. Be careful tochoose the correct orientation (look at the orientation scheme on the colorimeter, the beam is in the
direction of the yellow arrow, see figure below), and to push the cuvette down until the final
position. Close the lid.
Turn the selection wheel until a dashed line appears around “Absorbance” and then press the
OK button. Using the selection wheel, highlight “Calibration” and press the OK button.

Wait until the display reads 0.00 (or ‒0.00).
Place the cuvette with ca 3 cm in height of the analyzed solution in the tank. Close the lid.
Read the absorbance value.
CuvetteCuve tank

51st IChO – Practical Exam

Orientation
scheme16

XXX_1
I.

Determination of the iron content in the wine

In this part, you will need the 0.000200 M iron(III) solution and the 1 M potassium thiocyanate
solution.

Procedure
1.
Prepare 6 tubes by adding to each tube the required volumes of the provided solutions, as
described in the table below.
Tube #

5

6

1.0 mL

1.0 mL

Wine

10.0 mL

10.0 mL

Hydrogen peroxide
solution

0.5 mL

0.5 mL

0.000200 M iron(III)solution
1 M perchloric acidsolution

Deionized water
2.

1

2

3

4

1.0 mL

2.0 mL

4.0 mL

6.0 mL

1.0 mL

1.0 mL

1.0 mL

1.0 mL

9.5 mL

8.5 mL

6.5 mL

4.5 mL

1.0 mL

Stopper the tubes and homogenize.

3.
Add 1.0 mL of 1 M potassium thiocyanate solution in tubes 1, 2 3, 4 and 5. Do not add intube 6. Stopper and homogenize.
4.

When all the tubes are ready, raise your HELP card to get a colorimeter from a lab assistant.

5.
Prepare the colorimeter using the procedure described previously (see page 16). Set the
wavelength at 470 nm. Use deionized water for the blank.
6.
Record the absorbance of each tube (1 to 6) at this wavelength. Report the results in thefollowing table. Raise your HELP card to return the colorimeter.
Tube #

1

2

3

4

16

32

64

96

5

6

Absorbance (at 470 nm)
Analytical concentrationof Fe3+ in the tubec(Fe3+) / µM
Colorimeter code

51st IChO – Practical Exam17

XXX_1
Questions
Plot the absorbance A of tubes 1 to 4 as a function of the analytical concentration of Fe3+ in

1.

the tube.


In the following, check the boxes of the data you will consider for your calibration curve.

Tube #

1

2

3

4

Absorbance values used
for the calibration curve2.
Using the previous plot and the data you have chosen, draw the calibration straight line on
the previous plot determine the analytical concentration (in µmol L‒1) of Fe3+ in tube 5.

c(Fe3+)TUBE 5 =51st IChO – Practical Examµmol L‒118

XXX_1
If you could not calculate c(Fe3+), the value c(Fe3+) = 50 µmol L‒1 can be used in the rest of the
problem.
3.

Calculate the mass concentration, in mg per liter, of iron in the studied white wine.

cm(iron) =51st IChO – Practical Exammg L‒119

XXX_1II.

Determination of the complex stoichiometry

In this part, you will need the 0.00200 M iron(III) solution and the 0.00200 M potassium thiocyanate
solution.

Procedure
In part I of this problem, we use the color of the iron(III)-thiocyanate complex to determine theconcentration of iron in the sample of wine. Part II of this problem aims at investigating the
stoichiometry of the [Fea(SCN)b](3a‒b)+ complex (coordination of water is not shown), where a and b
are integers no greater than 3.
You are provided with the following aqueous solutions for this part:
0.00200 M iron(III) solution (already acidified) (80 mL)
0.00200 M potassium thiocyanate solution (80 mL)
You also have test tubes (with stoppers that you can wash and dry), graduated pipettes, aspectrophotometer cuvette, a colorimeter (upon request), and any other labware on your bench thatyou think useful.
1. Fill the first three lines of the following table with volume values that will allow you todetermine the stoichiometry of the complex, by spectrophotometric measurements. You don’t
have to fill all the columns. Calculate the molar fraction of iron(III) in each tube, using thefollowing formula.
𝑥(Fe3+ ) =Tube #

7

8

9

𝑉Fe(III)𝑉Fe(III) + 𝑉SCN−
10

11

12

13

14

15

Volume of 0.00200 M
iron(III) solution
VFe(III) / mL
Volume of 0.00200 M
potassium thiocyanate
solution VSCN‒ / mL
Molar fraction iniron(III) x(Fe3+)
Absorbance (at 470
nm)
Colorimeter code2.
Prepare the tubes. When all the tubes are ready, raise your HELP card to get a colorimeterfrom a lab assistant.
3.
Prepare the colorimeter using the procedure described previously (see page 16). Set the
wavelength at 470 nm. Use deionized water for the blank.
51st IChO – Practical Exam20

XXX_1
4.
Record the absorbance of each tube at this wavelength. Report the results in the previous
table.

Questions
4.

Plot the absorbance A of the tubes as a function of the molar fraction of iron(III) x(Fe3+).

5.
Based on the results of the experiments you carried out, determine the stoichiometry of thecomplex [(Fe)a(SCN)b](3a‒b)+.

a=

51st IChO – Practical Examb=

21

XXX_1
Problem
P3
13% oftotal

Question
PointsTitrationI
10

Titration
II
10

Titration
III
8

P3.1

P3.2

P3.3

P3.4

P3.5

Total

4

4

2

2

2

42

Score

Problem P3. Wine for keeping
Sulfur dioxide, SO2, is used as a preservative in wine. When SO2 is added to wine, it can react with waterleading to bisulfite ions, HSO3‒, and protons, H+. Bisulfite can also be converted to sulfite, SO32‒, by the lossof a second proton.
SO2 + H2O = H+ + HSO3‒
HSO3‒ = H+ + SO32‒
These three different forms of sulfur dioxide in water can react with chemicals in wine such as acetaldehyde,
pigments, sugars, etc. forming products P. The total concentration of sulfur dioxide is the sum of theconcentration of the “free” forms (SO2, HSO3‒ and SO32‒) and P.
The preservative concentration is regulated because sulfites and sulfur dioxide can be harmful to somepeople. In the EU, the maximum total sulfur dioxide content is set at 100 mg L‒1 for red wine and 150 mg L‒1
for white or rosé.
Your task is to determine the total sulfur dioxide concentration of the provided white wine byiodometric titration.

Procedure
I.

Standardization of the sodium thiosulfate solution

1.
You are given a sample of ca 100 mg of pure potassium iodate KIO3. The exact mass iswritten on the label of the vial. Report it in the table below.
2.
Prepare 100 mL of potassium iodate solution in the 100 mL volumetric flask, using thewhole sample of solid potassium iodate and deionized water. This solution is called S.
3.




In a 100 mL Erlenmeyer flask, add:
20 mL of solution S with a volumetric pipette;
5 mL of the potassium iodide solution (0.5 M), using a 5 mL graduated cylinder;
10 mL of the sulfuric acid solution (2.5 M) with a 10 mL graduated cylinder.

4.
Swirl the Erlenmeyer flask, cover it with Parafilm and keep it in the cupboard for at least
five minutes.
5.
Fill the burette with the provided thiosulfate solution using a beaker. Titrate the content of
the Erlenmeyer flask with constant swirling. When the liquid turns pale yellow, add ten drops of the
starch solution and keep titrating until the solution becomes colorless. Record the titration volume
V1.
6.

Repeat the procedure (steps 3-5) as needed.

51st IChO – Practical Exam22

XXX_1Mass of potassium iodate(report the value on the label)

Analysis n°

V1 / mL1
2
3

Reported value V1 / mL
II.

Standardization of the iodine solution

1.
With a volumetric pipette, transfer 25 mL of the iodine solution labeled I2 into a 100 mL
Erlenmeyer flask.
2.
Titrate the content of the Erlenmeyer flask with the sodium thiosulfate solution. When theliquid turns pale yellow, add ten drops of the starch solution and keep titrating until the solution
becomes colorless. Record the titration volume V2.
3.

Repeat the procedure (steps 1-2) as needed.

Analysis n°

V2 / mL1
2
3

Reported value V2 / mL
51st IChO – Practical Exam23

XXX_1
III.

Determination of total sulfur dioxide

1.

With a volumetric pipette, transfer 50 mL of wine into a 250 mL Erlenmeyer flask.

2.
Add 12 mL of the sodium hydroxide solution (1 M), with a 25 mL graduated cylinder.
Cover the flask with Parafilm, swirl the content then let it stand for at least 20 minutes.
3.
Add 5 mL of the sulfuric acid solution (2.5 M), and ca 2 mL of starch solution using a
graduated disposable plastic pipette.
4.
Titrate the content of the Erlenmeyer flask with the iodine solution in the burette, until adark color appears and persists for at least 15 seconds. Record the titration volume V3.
5.

Repeat the procedure (steps 1-4) as needed.

Analysis n°

V3 / mL1
2
3

Reported value V3 / mL

51st IChO – Practical Exam24

XXX_1
Questions1.
Write down the balanced equations of all the reactions occurring during the standardizationof the sodium thiosulfate solution.

2.
Calculate the molar concentration of the sodium thiosulfate solution. The molar mass ofpotassium iodate is M(KIO3) = 214.0 g mol‒1.

c(S2O32‒) =mol L‒1

If you could not calculate c(S2O32‒), the value c(S2O32‒) = 0.0500 mol L‒1 can be used in the rest of
the problem.
3.

Calculate the molar concentration of the iodine solution.

51st IChO – Practical Exam25

XXX_1c(I2) =

mol L‒1

If you could not calculate c(I2), the value c(I2) = 0.00700 mol L‒1 can be used in the rest of the
problem.
4.
Write down the equation of the reaction between iodine I2 and sulfur dioxide SO2, assumingthat sulfur dioxide is oxidized into sulfate ions SO42‒.

5.
Calculate the mass concentration, in mg per liter, of total sulfur dioxide in the wine. The
molar mass of sulfur dioxide is M(SO2) = 64.1 g mol‒1.

cm(SO2) =51st IChO – Practical Exammg L‒126

XXX_1PENALTIES
Incident #Student signature

Lab supervisor signature

1 (no penalty)
2

3

4

5

51st IChO – Practical Exam

27