11TH CHEMISTRY OLYMPIAD OF THE BALTIC STATES
EXPERIMENTAL EXAMINATION
1. SYNTHESIS OF A TRIBOLUMINISCENT SUBSTANCE
Triboluminiscence is a property of crystals to emit light when being crushed. A
substance exhibiting triboluminiscence is easily obtained by reacting 2-aminobenzoic acid with acetic anhydride. Triboluminiscence depends greatly on
crystal size. Larger crystals will result in brighter triboluminiscence1.
Reagents
2-Aminobenzoic acid (M = 137.14 g/mol)
Caution: 2-aminobenzoic acid is irritating.
Acetic anhydride (1.5 g contained in a disposable syringe, M = 102.09 g/mol)
Caution: acetic anhydride is corrosive, work only with gloves.
6 M hydrochloric acid
Caution: 6 M hydrochloric acid is irritating, work only with gloves.
60% ethanol
Caution: ethanol is flammable.
Distilled water
Chipped ice
Procedure
Weigh 1.00 g of 2-aminobenzoic acid and transfer it to a test tube. Add
3 ml of distilled water to the tube and stir well. Dropwise with stirring add
6 M hydrochloric acid to the mixture, until the amount of precipitate reaches
minimum and just starts to increase again. Dissolve the precipitate completely
by adding a few drops of distilled water to the reaction mixture.
Dropwise with stirring add to the reaction mixture all acetic anhydride
contained in the syringe. Dispose of the empty syringe in a tank with
5% sodium hydroxide, located in the hood. Heat the tube on a boiling water
bath2, until the precipitate becomes crystalline. Cool the tube in an ice bath.
Collect the precipitate by vacuum filtration with a fritted glass filter. Wash
the precipitate on the filter several times with small portions of distilled water,
and transfer it back into the tube, previously rinsed with distilled water.
Recrystallize the reaction product from 60% ethanol. Either let the hot
solution cool by itself, or place it in a vessel lined with cotton wool, or cool it
in an ice bath. After crystallization is complete, collect the crystals by vacuum
filtration and dry them carefully between sheets of filter paper.
Weigh the recrystallized compound and record the yield. Accompanied by
the lab instructor, proceed to a darkened room to observe triboluminiscence.
1
2
Larger crystals will also be given a higher grade.
Hot plate setting of 2-3 ensures smooth boiling.
Name ………………………………… Country ……………..
1. SYNTHESIS OF A TRIBOLUMINISCENT SUBSTANCE
ANSWER SHEET
1. Reaction equation
2. Why is hydrochloric acid added to the reaction mixture? Mark the only
correct answer.
a)
b)
c)
d)
Hydrochloric acid is an essential catalyst of the reaction.
Hydrochloric acid prevents decomposition of acetic anhydride.
Hydrochloric acid prevents decomposition of 2-aminobenzoic acid.
Hydrochloric acid is needed to dissolve 2-aminobenzoic acid.
3. Why the amount of precipitate goes through a minimum and then starts to
increase again on addition of hydrochloric acid? Mark the only correct
answer.
a) 2-Aminobenzoic acid decomposes on addition of excess
hydrochloric acid.
b) On addition of excess hydrochloric acid, 2-aminobenzoic acid forms
a salt, which is insoluble in water.
c) The common ion effect causes precipitation of 2-aminobenzoic acid
hydrochloride.
d) The common ion effect causes precipitation of 2-aminobenzoic acid.
4. Which approach facilitates formation of large crystals on recrystallization?
Mark the only correct answer.
a)
b)
c)
d)
Letting the hot solution cool by itself.
Cooling the hot solution in an ice bath.
Cooling the hot solution in a vessel lined with cotton wool.
Cooling conditions do not influence crystal size.
5. Record the mass of the recrystallized product and calculate the yield.
2. DETERMINATION OF THE MOLAR MASS OF A GAS
The universal gas equation provides a powerful tool for determination of
physical constants of a gas, including molar mass. The volume, mass, and
pressure of a gas can be easily measured, and this allows obtaining many
physical constants by simple calculation. Yet the actual experimental
conditions are often far from ideal, and neglect of them may completely
undermine the results of the experiment.
Materials
Balloon
Notes:
1. Inflate the balloon and let the air out several times before filling it with the unknown
gas.
2. Extra balloons are available at the expense of 2 points penalty per balloon.
Peace of rope (1.5 m)
The following glassware and equipment is also available in the lab:
Cylinder containing the unknown gas
Caution: the gas is under the pressure of 200 atm.
Notes:
1. The gas does not necessarily have to be a single substance.
2. The gas is essentially insoluble in water.
Electronic balances (±0.01 g)
Graduated glass cylinder (2 l)
Tub (15 l)
Glass stopcock with attached rubber tubing
Digital thermometer
Barometer
Table showing vapour pressure of water vs. temperature.
Ruler (25 cm)
Scissors
Procedure
You are free to devise a procedure that you think will allow achieving the
objective most accurately. Several different approaches are possible. The use
of all provided materials is not essential for success. The highest grade will be
given to the most accurate answer that will take into account the experimental
conditions.
VAPOUR PRESSURE OF WATER
Temp.,
o
C
16.0
16.2
16.4
16.6
16.8
17.0
17.2
17.4
17.6
17.8
18.0
18.2
18.4
18.6
18.8
Pressure,
Pa
1818
1841
1865
1889
1913
1937
1962
1987
2012
2038
2063
2090
2116
2143
2169
Temp.,
o
C
19.0
19.2
19.4
19.6
19.8
20.0
20.2
20.4
20.6
20.8
21.0
21.2
21.4
21.6
21.8
Pressure,
Pa
2197
2224
2252
2280
2309
2338
2367
2396
2426
2456
2486
2517
2548
2580
2611
Temp.,
o
C
22.0
22.2
22.4
22.6
22.8
23.0
23.2
23.4
23.6
23.8
24.0
24.2
24.4
24.6
24.8
Pressure,
Pa
2643
2676
2709
2742
2775
2809
2843
2877
2912
2948
2983
3019
3056
3093
3130
Temp.,
o
C
25.0
25.2
25.4
25.6
25.8
26.0
26.2
26.4
26.6
26.8
27.0
27.2
27.4
27.6
27.8
Pressure,
Pa
3167
3205
3243
3282
3321
3361
3401
3441
3482
3523
3565
3607
3650
3692
3736
Name ………………………………… Country ……………..
1. DETERMINATION OF THE MOLAR MASS OF A GAS
ANSWER SHEET
Record clearly all physical data obtained and show calculation of the molar
mass of the gas.
FUNDAMENTAL PHYSICAL CONSTANTS
2.99792458·108 m·s-1
Speed of Light in Vacuum
c
Permittivity of Vacuum
 
Elementary Charge
e
1.6021892·10-19 C
Planck Constant
h
6.626176·10-34 J·s
Avogadro Constant
NA
6.022045·1023 mol-1
Atomic Mass Unit
u
1.6605655·10-27 kg
Electron Rest Mass
me
9.109534·10-31 kg
Proton Rest Mass
mp
1.6726485·10-27 kg
Neutron Rest Mass
mn
1.6749543·10-27 kg
Faradey Constant
F
9.648456·104 C mol-1
Rydberg Constant
R∞
1.097373177·107 m-1
Bohr Radius
ao
5.2917706·10-11 m
Molar Gas Constant
R
8.31441 J·mol-1·K-1
Molar Volume, Ideal Gas
(T0 = 273.15 K, p0 = 1 atm)
VA
0.02241383 m3·mol-1

8.85418782·10-12 F·m-1