outer octet, aluminium chloride molecules dimerise forming Al2Cl6 . This being an example
of dative covalent bonding, where a bond is formed by the sharing of a pair of electrons
both of which are provided by a chlorine atom of another aluminium chloride molecule. The
diagram below illustrates the structure, where an arrow denotes a dative covalent bond
pointing from donor to acceptor:
Hence the molecular formula for the anhydrous aluminium chloride is Al2Cl6 , which
written in its empirical form is AlCl3.
Method and Materials:
Outline and Reactions:
The experiment involves several stages. First the aluminium chloride is added to
water and reacts forming aluminium oxide and dense white fumes of hydrogen chloride gas:
Al2Cl6 (s) + 3H2O(l) Al2O3(s) + HCl(g)
Since hydrogen chloride gas is very soluble, it dissolves to form hydrogen and chloride ions:
HCl(g) + H2O(l) H+(aq) + Cl- (aq)
Given the amphoteric nature of aluminium oxide, it reacts with the H+ ions as follows:
Al2O3(s) + H+(aq) 2Al3+(aq) + 3H2O(l)
In aqueous solution, the aluminium ion is stabilised by the co-ordination of a water molecule
to form the complex hexaaquaaluminium(III) ion ?Al(H2O) 6? 3+. The six water molecules
are distributed octahedrally about the Al3+ ion. The co-ordination of the water molecules
occurs by the donation of the lone pair of electrons on the oxygen atom. The O-H bonds
are also polar due to varying electronegativities. The following diagrams illustrate the
structure of the hexaaquaaluminium(III) ion and the polarity of the O-H bonds:
The partial positive charge on the hydrogen atoms, due to the higher electronegativity and
hence attraction on the electrons of oxygen, attracts bases, which may abstract protons from
the co-ordinated water molecules. In this particular case, water molecules itself may
functions as bases:
?Al(H2O) 6? 3+ + H2O ?Al(OH)(H2O) 5? 2+ + H3O+
?Al(OH)(H2O) 5? 2+ + H2O ?Al(OH)2(H2O)4? + + H3O+
Hence the solution becomes acidic. However, aluminium in the above form is undesired as
it reacts with the potassium chromate indicator forming aluminium chromate which is
greenish in colour, this makes the end-point fainter than it already is.
Thus if a strong base is added such as CO3 2- ions (in calcium carbonate), the hydroxonium
ions are removed, and the hydrolysis equilibria moves to the right, hence precipitating out the
aluminium ions in the complex.
The potassium chromate indicator is then added, and a standard solution of silver nitrate is
titrated into the reaction vessel. The silver ions first react with the chloride ions, in
preference to the chromate ions since silver chloride is less soluble than silver chromate,
forming a white precipitate of silver chloride:
Ag+(aq) + Cl-(aq) AgCl (s)
When all the chloride ions have reacted with the silver ions, one more drop of silver nitrate
will result in silver ions reacting with the chromate ions forming a faint reddish precipitate of
silver chromate:
2Ag+(aq) + CrO42-(aq) Ag2CrO4 (s)
Steps Taken:
For safety reasons, safety spectacles are worn at all times and the
experimental area is kept tidy.
1. One gram of aluminium chloride is to be weighed out. I have
chosen one gram rather than 0.5-1.0g since the percentage error
in weighing one gram on the same balance, is less than that of
weighing 0.5g. The aluminium chloride is placed in a stoppered weighing bottle using a
spatula. Precautions, including replacing the lid of the weighing bottle as soon as possible,
are taken to avoid exposure to air. This is as the aluminium chloride reacts with moisture in
the air leading to the evolution of hydrogen chloride gas. The evolution of hydrogen chloride
gas is both harmful and will result in inaccurate results since less chloride ions will be formed
in solution. The balance is very accurate, and is accurate to + 0.00005g
2. The solution of the chloride must be prepared in a clean, wide-mouthed jar, of
approximately 250cm3 in capacity, with a water tight lid. Approximately 40cm3 of distilled
water is then added to the jar, and then the one gram of aluminium chloride is dropped
gently into the jar. The lid is then replaced quickly to prevent the loss of hydrogen chloride
gas. The jar is gently swirled to dissolve all the aluminium chloride, and left fort ten minutes
until all the hydrogen chloride fumes have dissolved and thus cannot be seen.
3. After all the hydrogen chloride gas has dissolved, the solution is transferred to a 100cm3
volumetric flask using a funnel. The jar that was used is then rinsed out several times and
may be put back. Finally, the volume is made up to the calibration mark using distilled
water, making sure that the bottom of the surface meniscus is in line with the calibration
mark. The solution is then mixed well by repeated inversion.
4. Using a pipette and filler, 10 cm3 of the solution is measured out into a conical flask.
For greater accuracy, the pipette is first rinsed with the solution to be used and then it is
filled up, up to the calibration mark, again making sure that the bottom of the surface
meniscus is in line with the calibration mark. Their should also be no air bubbles in the
pipette as this will produce inaccurate results. The tip of the pipette is then momentarily
placed against the surface of the solution in the conical flask in order that the remaining
solution in the pipette goes into the conical flask. However, their will still be a little of the
solution remaining at the bottom of the pipette, yet this should not be ‘blown’ out as this
residue is taken into account when the instrument is calibrated.
5. Small portions of calcium carbonate are added to the conical flask until there is no more
effervescence and a small amount of unreacted powder remains.
6. Ten drops of potassium chromate indicator, which is yellow in colour, is then added to
the solution
7. A burette, of 50cm3 in capacity, is then clamped to a stand, whilst making sure that the
burette is vertical. 0.05 molar solution of silver nitrate is then acquired. Silver nitrate is
poisonous and can stain the skin, therefore necessary safety precautions must be taken. The
burette is first rinsed with the silver nitrate solution, for maximum accuracy, and is then filled
up to the calibration mark using a funnel to prevent spillage. Any air bubbles present must
be removed as these help accumulate our experimental error.
8. The silver nitrate solution is then titrated into the conical flask. This is done by squeezing
clippers at the bottom of the burette to allow the silver nitrate to flow. Care is taken to
prevent the silver nitrate from going on to the sides of the conical flask and making sure it
goes directly into the chloride solution. The conical flask is gently, but continuously swirled
throughout the titration. At first, a white precipitate of silver chloride appears, but the
titration is continued, with shaking, until the first permanent faint reddish tinge of silver
chromate is seen. The first titration is a trial run and gives us an indication of the volume of
titre required. The titration is then repeated with greater accuracy until, ideally, when
consecutive results agree to within 0.1cm3.
Results and Calculations
Results:
Mass of weighing bottle before, m1 +0.0005 4.688g
Mass of weighing bottle after, m2 +0.0005 5.688g
Mass of Aluminium Chloride, m1-m2 +0.001 1.000g
N.B.- The error is taken as the maximum error-absolute error-of the instrument. When
subtracting, i.e. finding the mass of aluminium chloride, the value of the absolute error in the
answer is the sum of the absolute errors in the two masses.
Pipette Solution: Chloride ions in solution
Burette solution: Silver Nitrate solution 0.05 mol dm-3
Indicator: Potassium Chromate
Trial 1 2 3
Initial 0.0 0.0 0.0 NA
Burette readings
Final 45.3 44.7 44.8 NA
Volume Used (titre)/cm3 +0.05 45.3 44.7 44.8 NA
Mean titre/cm3 +0.1 44.75
N.B.-The trial titration was not taken into account when taking the mean as it was only done
to obtain an approximate titre value. In addition to this, only two more titrations were
required as they were within 0.1cm3 of each other. Also, the error in the mean is
+0.1, since calculating the mean involves an addition and a division. When adding numbers,
the value of the absolute error in the answer is the sum of the absolute errors in the numbers.
The division does not add to the error as we are dividing by a finite number.
Calculations:
The following steps were taken in calculating the results:
1. The number of silver ions added from the burette are found-
This is done in two steps-
i) finding the number of moles of silver nitrate added from the burette
ii) using this to find the number of chloride ions added from the burette
i) Since Concentration = Number of moles
Volume
==> Number of moles = Concentration x Volume, this is written as C = nv
where C is the concentration in moles per cubic decimetre, n is the number of moles in
moles and v is the volume in cubic decimetres. v in this case is the mean titre.
? n AgNO3 (aq) = 0.05 x 44.75 = 2.2375 x 10-3 mol
1000
ii)From the stoichiometry of the equation, we find that the ratio n AgNO3 : n Ag+ is 1:1 :-
AgNO3 (aq) Ag+(aq) + NO3- (aq)
? n AgNO3 = n Ag+ = 2.2375 x 10-3 mol
2. The number of moles of chloride ions in the conical flask are found as follows:
The silver ions react with the chloride ions according to the following reaction:
Ag+(aq) + Cl-(aq) AgCl (s)
Hence from the stoichiometry, we find that the ratio n Ag+ : n Cl- is 1:1
? n Cl- = n Ag+ = 2.2375 x 10-3 mol
3. The mass of the chloride ions in the conical flask must then be calculated:
This is done by using the equation Mass = number of moles x molar mass, which is written
as m = nM. Given the number of moles of chloride ions is as above, and the molar mass of
chlorine is 35.5 atomic mass units:
? m Cl- = 2.2375 x 10-3 x 35.5 = 0.0794g
4. The mass of aluminium chloride in the conical flask is then calculated:
Given that 1.000g of anhydrous aluminium chloride was added to the wide mouthed jar and
made up to 100cm3, the mass in the conical flask, which has 10cm3 of this, is a tenth of the
initial mass, which is 0.1000g.
5. The mass of Aluminium in the conical flask is then found:
This is done by subtracting the mass of the chloride ions in the conical flask from the mass of
aluminium chloride in the conical flask:
m Al = 0.1000 - 0.0794 = 0.0206g
6. The number of moles of aluminium is then found:
This is done by using the equation Number of moles = mass i.e. n = m
molar mass M
The molar mass of aluminium is 27.0 atomic mass units,
? n Al = 0.0206 = 7.630 x 10-4 mol
27.0
7. The ratio moles of chloride ions is then found:
moles of aluminium ions
Since n Cl- = 2.2375 x 10-3 mol, and n Al = 7.630 x 10-4 mol
? n Cl- = 2.2375 x 10-3 = 2.937
n Al 7.630 x 10-4
8. With this information, the empirical formula of aluminium chloride, AlxCly, can now
be determined:
Since ratio n Cl : n Al is 1: 2.937, and given that elements react in simple, whole number
ratios, 2.937 may be taken as three,
? Empirical Formula Of AlxCly is AlCl3
Discussion and Evaluation
Thus, both in light of our results, and the theory presented in the introduction, we may
conclude that the empirical formula of the anhydrous aluminium chloride sample is AlCl3 .
Our results appear to have been of marked accuracy. The percentage error is found as
follows:
(3- 2.937) x 100 = 2.1%
3
This percentage error, given the laboratory conditions, is rather lower than expected as there
are many probable sources of error, of varied significance, in this experiment.
There are two main types of error that may have been encountered:
i) Systematic errors- which include faulty apparatus, contaminated samples and badly
calibrated instruments. Repeating the measurement will have no effect on this type of
error and may only become evident after the final result is calculated.
ii) Random errors- which depend on the experimenters ability to use the apparatus. In this
case, making a number of readings of a given quantity, i.e. repeating a titration, and
taking an average will reduce the overall error.
In light of our results, it appears that the systematic error is negligible, or at least negligible
with respect to the random error. One example of a systematic error, of very little bearing
on the outcome of the experiment, is the initial composition of the aluminium chloride, which
is according to the table obtained from the chemical company, as follows:
Product specification: ANHYDROUS ALUMINIUM CHLORIDE:
CHEMICAL PROPERTIES
Aluminium Chloride
99.0% min
Water insolubles
0.1% max
Non volatiles
1% max
Aluminium
20% min
Chlorine
79% min
Free aluminium
150ppm max
Iron
100ppm max
Zinc
30ppm max
Nickel
20ppm max
Lead
20ppm max
Copper
10ppm max
Chromium
10ppm max
Potassium
10ppm max
As, the above table shows, at least 99.0% of the sample is aluminium chloride. One other
possible systematic error may have been the accuracy of the standard solution of silver
nitrate, however it is likely to have been of high accuracy in light of our results.
More significantly, are the random errors that may have occurred. The many measurements
that were taken involved considerable individual judgement, such as making sure the bottom
of the surface meniscus was in line with the calibration mark, and judging when the end-point
had been attained, which in this case was quite faint.
Looking at the errors individually may be of some benefit. The first reading that was taken
was that of the mass of aluminium chloride, given that we had stated earlier that the accuracy
was + 0.001g, the percentage error in the measurement of 1g is:
0.001 x 100 = 0.1 %
1
An additional error may have been encountered in transferring the aluminium chloride into
the wide mouthed open-jar, as some residue is likely to have remained in the weighing bottle
and on the spatula. In order to limit this, even though of very small significance, the mass of
the bottle before and after removing the aluminium chloride should have been noted and
hence we would have known the exact mass of aluminium chloride added to the jar. Some
of the aluminium chloride may have also reacted with moisture in the air, however, as
exposure to the air was kept to a minimum and no hydrogen chloride gas was seen evolved,
this too is of negligible importance.
Measuring out a 100cm3 of water in the volumetric flask and pipetting this into the conical
flask is likely to have been of high accuracy. Yet the source of error that is most likely to
have contributed mostly to the 2.1% error attained, was the measurement of the titre. The
burette was accurate to the nearest millimetre, and hence the error in the mean is:
0.1 x 100 = 0.22 %
44.75
Although this is very accurate, there are additional errors that were involved in this
measurement, notably judging the end-point of the titration. A little of the titre ended-up on
the sides of the conical flask and the change from a white precipitate to a permanent faint
reddish tinge of silver chromate is indeed hard to judge perfectly, as white to faint red is not
very distinct and in addition, the white was already changing to faint-red, but this was not
permanent. However, the fact that this was repeated, reduced the likely error. Using a
burette accurate to a tenth of a millimetre, if there is one for the volume used, would be a
useful improvement to the method.
In conclusion the results attained, given that there were no anomalous errors and the
experimental error was very low, were very satisfactory, and have verified, to a reasonable
extent, the theory in the introduction stating that the empirical formula of the aluminium
chloride is AlCl3. Another related experiment may be designed to determine whether or not
the molecular formula of aluminium chloride is Al2Cl3.