THE COMPETITION PROBLEMS FROM THE
INTERNATIONAL CHEMISTRY OLYMPIADS Volume 1
1
st
– 20

THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Editor: Anton Sirota
ISBN 978-80-8072-082-7
Copyright © 2008 by IUVENTA – ICHO International Information Centre, Bratislava, Slovakia
You are free to copy, distribute, transmit or adapt this publication or its parts for unlimited teaching purposes,
however, you are obliged to attribute your copies, transmissions or adaptations with a reference to "The
Competition Problems from the International Chemistry Olympiads, Volume 1" as it is required in the
chemical literature. The above conditions can be waived if you get permission from the copyright holder. Issued by IUVENTA in 2008
with the financial support of the Ministry of Education of the Slovak Republic

Number of copies: 250
Not for sale. International Chemistry Olympiad

ICHO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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11
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1
Preface

This publication contains the competition problems from the first twenty International
Chemistry Olympiads (ICHO) organized in the years 1968 – 1988. It has been published
by the ICHO International Information Centre in Bratislava (Slovakia) on the occasion of
the 40th anniversary of this international competition.
Not less than 125 theoretical and 50 practical problems were set in the ICHO in the
mentioned twenty years. In the elaboration of this collection the editor had to face certain
difficulties because the aim was not only to make use of past recordings but also to give
them such a form that they may be used in practice and further chemical education.
Consequently, it was necessary to make some corrections in order to unify the form of the
problems. However, they did not concern the contents and language of the problems.
Many of the first problems were published separately in various national journals, in
different languages and they were hard to obtain. Some of them had to be translated into
English. Most of the xerox copies of the problems could not be used directly and many
texts, schemes and pictures had to be re-written and created again. The changes concern
in particular solutions of the problems set in the first years of the ICHO competition that
were often available in a brief form and necessary extent only, just for the needs of
members of the International Jury. Some practical problems, in which experimental results
and relatively simply calculations are required, have not been accompanied with their
solutions. Recalculations of the solutions were made in some special cases ony when the
numeric results in the original solutions showed to be obviously not correct. Although the
numbers of significant figures in the results of several solutions do not obey the criteria
generally accepted, they were left without change.
In this publication SI quantities and units are used and a more modern method of
1
11
1
st
stst
st

4 theoretical problems
2 practical problems


% compared with that of the starting mixture and the resulting mixture had the composition
as follows: 60 volume % of chlorine, 10 volume % of hydrogen, and 30 volume % of
hydrogen chloride.
Problems:
1.1 What is the composition of the initial gaseous mixture?
1.2 How chlorine, hydrogen, and hydrogen chloride are produced?
____________________ SOL UTI ON
1.1 H
2
+ Cl
2
→ 2 HCl
30 volume parts of hydrogen chloride could only be formed by the reaction of 15
volume parts of hydrogen and 15 volume parts of chlorine. Hence, the initial
composition of the mixture had to be:
Cl
2
: 60 + 15 = 75 %
H
2
: 10 + 15 = 25 %

1.2 Chlorine and hydrogen are produced by electrolysis of aqueous solutions of
NaCl: NaCl(aq) → Na
+
(aq) + Cl
-

THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia
6

PROBLEM 2
Write down equations for the following reactions:
2.1 Oxidation of chromium(III) chloride with bromine in alkaline solution (KOH).
2.2 Oxidation of potassium nitrite with potassium permanganate in acid solution (H
2
SO
4
).
2.3 Action of chlorine on lime water (Ca(OH)
2
) in a cold reaction mixture.
____________________

S OLU T I ON
2.1 2 CrCl
3
+ 3 Br
2
+ 16 KOH → 2 K

2
→ CaOCl
2
+ H
2
O THE 1
ST
INTERNATIONAL CHEMISTRY OLYMPIAD, 1968
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia
7PROBLEM 3
The gas escaping from a blast furnace has the following composition:
12.0 volume % of CO
2
28.0 volume % of CO
3.0 volume % of H
2

2
14
2 H
2
+ O
2
→ 2 H
2
O 1.5
CH
4
+ 2 O
2
→

CO
2
+ 2 H
2
O 1.2
C
2
H
4
+ 3 O
2
→

2 CO
2

of the gas
20.76
x
4 = 83.04 parts of N
2
for 100 m
3
of the gas THE 1
ST
INTERNATIONAL CHEMISTRY OLYMPIAD, 1968
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia
8 Balance: CO
2
H
2
O N
2
O
2

ST
INTERNATIONAL CHEMISTRY OLYMPIAD, 1968
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia
9

PROBLEM 4

A volume of 31.7 cm
3
of a 0.1-normal NaOH is required for the neutralization of
0.19 g of an organic acid whose vapour is thirty times as dense as gaseous hydrogen.
Problem:
4.1
Give the name and structural formula of the acid.
(The acid concerned is a common organic acid.)
____________________

SOL UTI ON

4.1
a) The supposed acid may be: HA, H
2
(acid)
(acid)
(acid)
M
m
n
=

1
3
molg60
mol1017.3
g19.0
(acid)
−
−
×=
×
×= vvM (1)

b) From the ideal gas law we can obtain:

1
1
2 2
M
M
ρ
PROBLEM 1
(Practical)
There are ten test tubes in the rack at your disposal (1 – 10) and each test tube
contains one of aqueous solutions of the following salts: Na
2
SO
4
, AgNO
3
, KI, Ba(OH)
2
,
NH
4
Cl, Ag
2
SO
4
, Pb(NO
3
)
2

4
OH, CuSO
4
, KMnO
4
, FeCl
3
, KCl, and distilled water.

2
22
2
nd
ndnd
nd

_______________________________________________________________________ THEORETICAL PROBLEMS
PROBLEM 1

An amount of 20 g of potassium sulphate was dissolved in 150 cm
3
of water. The
solution was then electrolysed. After electrolysis, the content of potassium sulphate in the
solution was 15 % by mass.
Problem:
What volumes of hydrogen and oxygen were obtained at a temperature of 20 °C and a
pressure of 101 325 Pa?
____________________

SOL UTI ON
On electrolysis, only water is decomposed and the total amount of potassium
sulphate in the electrolyte solution is constant. The mass of water in the solution:
1.1
Before electrolysis (on the assumption that
ρ
= 1 g cm
-3
): m(H
2
O) = 150 g

) = 1.02 mol

THE 2
ND
INTERNATIONAL CHEMISTRY OLYMPIAD, 1969
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia

13-1 -1
2
2
(H ) 2.04 mol 8.314 J mol K 293.15 K
(H )
101325 Pa
n RT
V
p
× ×
= =

≈ 0.049 m
3
, resp. 49 dm

Problem:
2.1
What are the stoichiometric formulas of the compounds?
2.2
Write the corresponding chemical equation.
____________________

SOL UTI ON
2.1
Compound
A
:
K
x
N
y
O
z
16
47.48
14
13.85
39.1
38.67
z:y:x ===
= 0.989 : 0.989 : 2.968 = 1 : 1 : 3
A
: KNO
3
Compound

INTERNATIONAL CHEMISTRY OLYMPIAD, 1969
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia

15
PROBLEM 3
A 10 cm
3
sample of an unknown gaseous hydrocarbon was mixed with 70 cm
3
of
oxygen and the mixture was set on fire by means of an electric spark. When the reaction
was over and water vapours were liquefied, the final volume of gases decreased to 65
cm
3
. This mixture then reacted with a potassium hydroxide solution and the volume of
gases decreased to 45 cm
3
.
Problem:
What is the molecular formula of the unknown hydrocarbon if volumes of gases were
measured at standard temperature and pressure (STP) conditions?
____________________

SOL UTI ON



Consumed in the reaction:
0.025
22.4
mol of O
2

According to the equation:
C
x
H
y
+ (x +
y
4
) O
2
= x CO
2
+
y
2
H
2
O
Hence,
0.020
22.4
mol of O
2

THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia

163
n
(C) =
n
(CO
2
) =
0.020
22.4
mol
n
(H
2
) = 2
n
(H
2
O) =
0.020
22.4
mol
x : y =
n


THE 2
ND
INTERNATIONAL CHEMISTRY OLYMPIAD, 1969
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia

17PROBLEM 4
Calcium carbide and water are the basic raw materials in the production of:
a) ethanol
b) acetic acid
c) ethylene and polyethylene
d) vinyl chloride
e) benzene

Problem:
Give basic chemical equations for each reaction by which the above mentioned
compounds can be obtained. SOL UTI ON

Basic reaction: CaC

O
CH
OH
vinyl alcohol
rearrangement
CH
O
reduction
CH
2
OH
acetaldehyde
ethanolb) acetic acid CH
CH
CH
2
CH
3
CH
3
COOH
acetic acid
HgSO
4


CH
CH
CH
2
catalyst
+ H
2
O
CH
2
ethylene
CH
2
CH
2
pressure, temperature
catalyst
( - CH
2
- CH
2
- )
n

polyethylene

PRACTICAL PROBLEMS

PROBLEM 1
(Practical) a) Three numbered test-tubes (1-3) contain mixtures of two substances from the
following pairs (4 variants):
1. ZnSO
4
- NaBr NaCl - Ca(NO
3
)
2
MgSO
4
- NH
4
Cl
2. AlCl
3
- KBr CaCl
2
- NaNO
3

- ZnSO
4
Al(NO
3
)
3
- NaCl

b) Each of the test-tubes numbered 4 and 5 contains one of the following substances:
glucose, saccharose, urea, sodium acetate, oxalic acid.

Problem:
By means of reagents that are available on the laboratory desk determine the
content of the individual test-tubes. Give reasons for both the tests performed and your
answers and write the chemical equations of the corresponding reactions.

Note:
For the identification of substances given in the above task, the following reagents
were available to competing pupils: 1 N HCl, 3 N HCl, 1 N H
2
SO
4
, concentrated H
2
SO
4
,
FeSO
4
, 2 N NaOH, 20 % NaOH, 2 N NHPROBLEM 2
(Practical) Allow to react 10 cm
3
of a 3 N HCl solution with the metal sample (competing pupils
were given precisely weighed samples of magnesium, zinc or aluminium) and collect the
hydrogen evolved in the reaction in a measuring cylinder above water. Perform the task by
means of available device and procedure.
In order to simplify the problem, calculate the mass of your metal sample from the
volume of hydrogen on the assumption that it was measured at STP conditions.
THE 3
RD
INTERNATIONAL CHEMISTRY OLYMPIAD, 1970
THE COMPETITION PROBLEMS FROM THE INTERNATIONAL CHEMISTRY OLYMPIADS, Volume 1
Edited by Anton Sirota,
ICHO International Information Centre, Bratislava, Slovakia

22THE THIRD
INTERNATIONAL CHEMISTRY OLYMPIAD
1–5 JULY 1970, BUDAPEST, HUNGARY

_______________________________________________________________________ THEORETICAL PROBLEMS PROBLEM 1
An amount of 23 g of gas (density
ρ
= 2.05 g dm

= =

mol1
molg44
g44
)(CO
1
2
==
−
nn
(C) = 1 mol
m
(C) = 12 g

mol1.5
molg18
g27
O)(H
1
2
==
−
n

n
(H) = 3 mol