Central exam
SwissChO 2015
Instructions

Write your name on each page and number these.

You have three hours to solve the problems. Wait for the START signal before you
begin.

Use a new page for each problem.

Write all necessary calculations legibly.

Put your pages into the provided envelope at the end of the exam. Do not stick down
the envelope.

Finish your work immediately when the STOP signal is given.

Leave your seat only when allowed to do so.

Only answers written on the answer sheets can be considered.

This test has 18 pages.
Viel Erfolg!
Bonne chance!
Buona fortuna!
Good luck!
Version: 2015.1
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Central exam
SwissChO 2015
Constants and formulae
Avogadro
constant:
NA = 6.022·1023 mol–1
Ideal gas
equation:
pV = nRT
Gas constant:
R = 8.314 J·K–1·mol–1
Gibbs energy:
G = H – TS
Faraday constant:
F = 96485 C·mol–1

 r G   RT ln K  nFE Zelle
Planck constant:
h = 6.626·10–34 J·s
Nernst equation:
Velocity of light:
c = 3.000·108 m·s–1
Energy of a
photon:
E
Temperature:
0 °C = 273.15 K
Lambert-Beer law:
A  log
hc

I0
 cL
I
For the calculation of equilibrium constants all concentrations refer to the standard
concentration 1 mol/dm3. Consider all gases ideal throughout this test.
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SwissChO 2015
Periodic table with relative atomic masses
1
18
1
2
H
He
1.008
2
13
14
15
16
17
4.003
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
Na
Mg
Al
Si
P
S
Cl
Ar
22.99
24.30
26.98
28.09
30.97
32.06
35.45
39.95
3
4
5
6
7
8
9
10
11
12
19
20
21
22
23
24
25
26
27
28
29
30
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
72.64
74.92
78.96
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
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
85.47
87.62
88.91
91.22
92.91
95.96
-
101.07
102.91
106.42
107.87
112.41
114.82
118.71
121.76
127.60
126.90
131.29
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
57-71
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
178.49
180.95
183.84
186.21
190.23
192.22
195.08
196.97
200.59
204.38
207.2
208.98
-
-
-
104
105
106
107
108
109
110
111
Rf
Db
Sg
Bh
Hs
Mt
Ds
Rg
-
-
-
-
-
-
-
-
55
56
Cs
Ba
132.91
137.33
87
88
Fr
Ra
-
-
89-103
Version: 2015.1
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
138.91
140.12
140.91
144.24
-
150.36
151.96
157.25
158.93
162.50
164.93
167.26
168.93
173.05
174.97
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
-
232.04
231.04
238.03
-
-
-
-
-
-
-
-
-
-
-
English
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Central exam
SwissChO 2015
Problem 1 – Multiple choice
7 Points
Even if the question is put in the singular one or more answers may be correct.
a)
In an experiment magnesium reacts with a diluted solution of hydrogen chloride:
Mg(s) + 2HCl(aq)
MgCl2(aq) + H2(g)
By which measurement during the process of the reaction could the rate of reaction be
determined best?
A
the mass of Mg
b)
C
the concentration of
MgCl2
D
the volume of H2
Which of the central atoms in the following species obeys the octet rule?
A
c)
B
the pH value of
the solution
NO
B
BH4-
C
PCl5
D
BF3
E
XeF4
Which of the following statements about oxygen containing compounds is correct?
A
KNO3 is a salt without oxidizing ability
B
Oxygen in PbO2 has the oxidation number -1, thus it is a peroxide
C
C and Si are in the same group so that both, CO2 and SiO2, are gaseous at
298 K
d)
D
The oxidation number of oxygen in OF2 is +2
E
H2O2 has oxidizing but no reducing properties
All compounds below show an acidic reaction in aqueous solution. Which of them does
not react as a Brönsted acid?
A
e)
H2SO4
B
C
H3BO3
D
H4SiO4
E
H2CrO4
HeH+ molecular ions are formed in a hydrogen-helium gas mixture under electron
impact conditions. Which of the following dissociation processes is characterized by
the smallest dissociation energy?
A
HeH+  He + H+
f)
H3PO4
B
HeH+  He+ + H
C
D
+
2+
- A and B
HeH  He + H
½ N2(g) + O2(g)

NO2(g)
K1
2 NO2(g)

N2O4(g)
K2
The two reactions given above have the equilibrium constants K1 and K2, respectively.
What would be the expression for the following reaction in terms of K1 and K2?
N2O4(g)

N2(g) + 2 O2(g)
A
Version: 2015.1
K1K2
B
K12K2
C
K1K22
English
D
1/K1K22
E
1/K12K2
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Central exam
g)
SwissChO 2015
Which of the following carbocations would you expect to rearrange
A
Version: 2015.1
B
C
English
D
E
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Problem 2 – Protolysis
6 Points
1.00 g NH4Cl and 1.00 g Ba(OH)2·8H2O are mixed and dissolved in 80 mL water. Then enough
water is added to the solution to get a total volume of 100 mL. NH3 is produced but remains in
the solution. Note the pKa of NH4+ ion: pKa = 9.24.
a)
Calculate the pH of the final solution.
b)
Calculate the molar concentration of all ions in the final solution
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Problem 3 – Stoichiometry
5 Points
Chlorine gas Cl2 is dissolved in a hot solution of KOH 12 M, and reacts to produce a mixture
of KCl, KClO3 and H2O.
a)
b)
c)
d)
Write down the equation of the reaction.
What volume of Chlorine gas at 25 °C and 1 bar must be introduced in the solution for
producing 100 g KClO3 in solution? (if you did not know the answer to a, assume that
for 1 mol of KClO3 5 mol Cl2 are needed)
What is the volume of the used KOH solution? (if you did not know the answer to a,
assume that for 1 mol of KClO3 4 mol KOH are needed)
At the end of the operation, the solution is cooled down to 0 °C, where 100 mL water
can only dissolve 3.1 g KClO3. Which mass of insoluble KClO3 is formed during the
cooling operation?
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Problem 4 – Titration
4 Points
The concentration of fluoride ions can be determined with boric acid (H3BO3) in an acidic
medium, due to the formation of HBF4. The quantity of fluoride ions in an unknown sample
needs to be determined. In order to do so 15.689 g of the sample are dissolved in deionized
water to obtain 100 mL of sample solution. 5 mL of this solution are then titrated with boric acid
with a concentration of 0.478 mol/L. For the titration an average of 13.56 mL boric acid are
needed.
a)
Write down the equation of the reaction.
b)
What is the percentage by mass of fluoride in the unknown sample? If you could not
solve question a) assume that one molecule of boric acid reacts with two fluoride ions.
Version: 2015.1
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Central exam
SwissChO 2015
Problem 5 – Thermochemistry
3 Points
A sample of metallic magnesium weighing 0.486 g is put in a beaker containing 100 mL HCl 2
M at 25 °C. A vigorous reaction starts producing some H2 gas and some Mg2+ ions in the
solution. If the heat of formation of the ion Mg2+ is -462 kJ/mol, calculate the final temperature
of the solution. The heat capacity of the solution is 4.185 J/g/K.
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Problem 6 – Equilibrium of an interhalogen compound
6 Points
Halogens form a series of interhalogens which are more or less stable. One of these is bromine
chloride, BrCl, which decomposes at 500 ºC into the elements. The equilibrium constant at this
temperature is Kc = 32 related to the decomposition of 2 units BrCl.
Let´s examine a certain reaction system, called system I, with c(BrCl) = c(Br2) = c(Cl2) = 0,25
mol/L
a)
b)
c)
d)
e)
f)
Write the equation for the decomposition.
Show by calculation this system is not at equilibrium.
In which direction will system I proceed?
Calculate Kp for this reaction.
Calculate the equilibrium concentrations of BrCl, Br2, and Cl2 in system I.
G is linked to Gº through the equation G = Gº + RT lnQ, Q being the reaction
quotient.
Keeping this in mind, what is the free enthalpy for the decomposition reaction in
system I?
Version: 2015.1
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Central exam
SwissChO 2015
Problem 7 – Chromate pollution
4 Points
A certain quantity of lead chromate (PbCrO4) was accidentally spilled into a reservoir, and the
city engineers would like to know to what extent drinking water was contaminated. The
solubility product Ksp of lead chromate at 18 ºC is 1.77 x 10-14.
a)
What is the solubility of lead chromate in pure water?
b)
Some engineers believe that the lead in the water could be removed by treating it with
potassium chromate K2CrO4. What is the solubility of lead chromate in 0.1 mol/L of
potassium chromate solution?
c)
It was also believed that chromate ions could be removed from potable water by
treating it with lead nitrate. What is the solubility of lead chromate in a 3.0 x 10 -7 mol/L
solution of lead nitrate?
Version: 2015.1
English
11/18
Central exam
SwissChO 2015
Problem 8 – Beer-Lambert
3 Points
The concentration of a colored substance in solution can be calculated using the LambertBeer’s law. A measurement of dye X with c(X) = 4.78 * 10-5 mol/L gave an absorption of 0.738.
For the measurement a cuvette with a length of 0.5 cm was used. For a sample with an
unknown concentration of X an absorption of 0.395 was measured.
a)
What is the concentration of X in the sample?
By an accident a second dye Y was added to a solution of dye X. The solution of dye X had a
concentration of 3.21 * 10-5 mol/L. A measurement of absorption was made to determine the
concentration of dye Y in the mixture.
b)
Calculate the concentration of Y, if A = 0.964 and εB = 5’838 l * cm / mol.
Version: 2015.1
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Central exam
SwissChO 2015
Problem 9 – VSEPR
7 Points
Draw the three-dimensional structures including the free electron pairs of the following
molecules in consideration of VSEPR theory:
a)
NH3
b)
H2O
c)
NO2-
d)
PCl5
e)
COCl2
f)
ClF3
g)
NO2+
h)
SF6
Version: 2015.1
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Central exam
SwissChO 2015
Problem 10 – Organic chemistry
4 Points
C2H4 is treated with HBr to produce a new substance A. Then A is reacted with NaOH to
produce a new substance B (C2H6O).
Then B is oxidized with an acidic solution of KMnO4 to produce C (C2H4O2).
Then another sample of B is mixed with C in anhydrous condition with a catalytic amount of
an acid to produce D (C4H6O2)
Determine the Lewis structures of A, B, C and D.
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Central exam
SwissChO 2015
Problem 11 – About organic acids
10 Points
a)
How many constitutional carboxylic acids exist, which satisfy the overall formula
C5H10O2?
Draw and name all such compounds. Indicate with asterisks at chiral atoms, which
structures are optically active.
b)
Rank the above carboxylic acids in decreasing order of their expected pKa values in
aqueous solutions. Explain your reasoning for the ordering.
c)
If 18O labelled 1-propanol is reacted with acidified, unlabelled ethanoic acid, the isotopic
label is found to reside entirely within the organic product. Draw this product, and write
a generalized mechanism for the carboxylic acid/alcohol reaction.
d)
If, instead, the reaction employs unlabelled propanol and 18O labelled ethanoic acid,
show the products expected:
i)
for reaction occurring in 1 mol/L HCl and
ii)
for reaction initially in 1 mol/L NaOH, followed by acidification with 1 mol/L DCl.
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Central exam
SwissChO 2015
Problem 12 – Reduction and oxidation of organic compounds
9 Points
Reductions
a)
b)
c)
Oxidations
d)
e)
f)
g)
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SwissChO 2015
h)
i)
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Problem 13 – Reaction of free radicals
3 Points
Tert-butylperoxide is a stable easy to handle liquid which serves the formation of free radicals
through the following equation.
(CH3)3CO–OC(CH3)3
2 (CH3)3CO
A mixture of methylpropane and CCl4 is fairly stable at 130-140 ºC. By adding a small amount
of tert- butylperoxide a reaction occurs which leads to tert-butylchloride and chloroform.
Besides, some tert- butanol can be found which is twice the amount of peroxide added.
Give all steps of a possible mechanism for this reaction. Start with the reaction of the free
radical and isobutane.
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