Introduction
The salt copper(II) sulfate is a hydrous salt which means that it constitutes water. In many
cases, the water component of the salt is not required for many of its reactions. Water leaves
the crucible in the form of water vapor. In this experiment, we would like to investigate how
many moles of water are present in the hydrous copper(II) sulfate salt. To get the anhydrous
form of Copper (II) Sulfate, we must heat up the crucible so that the water can escape the
crucible.
The chemical equation for this reaction is given by:
𝐶𝑢𝑆𝑂4 • 𝑥𝐻20 → 𝐶𝑢𝑆𝑂4 + 𝑥𝐻20
According to the stoichiometric equation, the value of x is being investigated.
Apparatus Used
-
Crucible with Lid
Tripod Stand
Bunsen Burner
Lighter
Electronic Mass Balance
Porcelain Triangle
Chemicals Used: Hydrous Copper (II) Sulfate
Experimental Procedure
For this experiment, we will only be measuring masses. A dry crucible with its lid on is
weighed on the electronic mass balance. Then, approximately 1 gram of Copper (II) Sulfate is
added to the crucible and this is measured again. This reading is the crucible with the hydrous
Copper (II) Sulfate.
Using crucible tongs, the closed crucible with lid should be placed on the tripod using the
porcelain triangle. Then we heat the crucible with gentle bunsen flame for about two minutes
with the lid covering half of the crucible. After 2 minutes of heating the crucible, the crucible
should be heated strongly while removing the crucible lid completely. This will ensure a
faster rate of water vapor leaving the crucible at the backend of heating.
During the course of vigorous heating, we can observe the color of the anhydrous salt and
make conclusions. Take out the crucible using tongs, let it cool, and place the crucible in the
electronic mass balance. Record down the mass and place the crucible back on the tripod
stand and heat it up for a few seconds. After the second round of heating, let the crucible
cool, place the crucible in the electronic mass balance, and record the mass. If this mass is the
same as the previous mass, then it would be certain that all the copper (II) sulfate has
completely turned anhydrous. During the process of cooling, the crucible is covered with the
lid as there is a possibility that the copper (II) sulfate observes water vapor from the
surroundings since it is hygroscopic.
Safety Precautions
The entire experiment should be conducted while wearing goggles and a lab coat. Copper (II)
Sulfate is a toxic irritant and one should avoid contact with the salt. Next, as this experiment
involves intense heating of the crucible, handling of the crucible should be done with crucible
tongs so that the person’s hands do not get burnt. Before lighting a bunsen burner, the air hole
should be closed in order to prevent any strike backs.
Qualitative Data Analysis
Figure 1. Picture of the crucible before
heating the copper (II) sulfate
Figure 2. Picture of the crucible after
heating the copper (II) sulfate
Observations
For the first picture, it can be observed that the hydrous copper (II) sulfate is bright blue in
color. For the second picture, it is observed that the anhydrous copper (II) sulfate is white in
color. During the heating process, steam was evolved from the crucible without odor, which
according to literature knowledge is said to be water vapor. Towards the end of heating, there
were also white fumes leaving the crucible. At the end of heating the crucible, black speckles
were spotted in the substance remaining in the crucible.
Explanation and Scientific Reasoning
The initial substance in the crucible before heating is Copper Pentahydrate, which is also
known as Copper (II) sulfate. When the crucible was heated, the water evaporated and left the
crucible. With the removal of water molecules, the copper (II) sulfate becomes a transition
metal salt. Hence, the color changed from blue to white.
Towards the end of the heating process, there were also white fumes that evolved from the
crucible. According to literature knowledge, this is supposed to be sulfur trioxide which is
formed when copper (II) sulfate decomposes. At high temperatures and after intense heating,
copper (II) sulfate decomposes into Copper (II) oxide (CuO) and Sulfur Trioxide (SO3).
This is the equation of the thermal decomposition of the salt. Hence, this explains
observations of the white fumes escaping the crucible where the fumes are the sulfur trioxide
gas and the black speckled spots are the Copper (II) Oxide. Hence, the final substance
observed in the crucible is grey in color.
Quantitative Data and Analysis
Sample Calculations
For the sample calculations, we will be using the first trial of calculations which will be
helpful to propagate uncertainties.
For finding the mass of water of crystallization, we have undertaken the assumption that
Mass of Crucible + Copper (II) Sulfate before Heating = 33.614 g
Mass of Crucible + Copper (II) Sulfate after Heating = 32.986 g
Mass of Water of Crystallization = 33.614 g - 32.986 g = 0.628 g
Taking molar mass of water to be 18.02 g/mol,
Molar Mass of H2O = 18.02 g/mol
No. of Moles of Water of Crystallization =
0.628 𝑔
18.02 𝑔/𝑚𝑜𝑙
= 0.0349 mol (3 s.f)
To find the mass of Anhydrous Copper (II) Sulfate, we have to find the difference between
the masses of the empty crucible and the crucible + Copper (II) Sulfate after heating. The
molar mass of Copper (II) Sulfate is 159.61 g/mol according to the IB Periodic table.
Mass of Anhydrous Copper (II) Sulfate = 32.986 g - 32.009 g = 0.977 g
No. of Moles of Anhydrous Copper (II) Sulfate =
0.977𝑔
159.61 𝑔/𝑚𝑜𝑙
= 0.00612 mol
To find the number of moles, we reference our calculations to the previous reaction equation.
𝐶𝑢𝑆𝑂4 • 𝑥𝐻2𝑂 → 𝐶𝑢𝑆𝑂4 + 𝑥𝐻2𝑂
Mole Ratio between CuSO4 and H2O = 1:x
0. 00612 𝑚𝑜𝑙 𝑜𝑓 𝐶𝑢𝑆𝑂4 → 0. 0349 𝑚𝑜𝑙 𝑜𝑓 𝐻2𝑂
1 𝑚𝑜𝑙 𝑜𝑓 𝐶𝑢𝑆𝑂4 →
0.0349 𝑚𝑜𝑙
0.00612
𝑜𝑓 𝐻2𝑂
Hence, 1 mol of CuSO4 yields 5. 70 𝑚𝑜𝑙 of H2O
𝑇ℎ𝑢𝑠,
𝑥 = 5. 70 (3 𝑠. 𝑓.)
Propagation of Uncertainties and Final Answer Presentation
𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 = 0. 001𝑔 + 0. 001 𝑔 = 0. 002 𝑔
% 𝑢𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑜𝑓 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 =
0.002𝑔
0.628 𝑔
× 100% = 0.
% 𝑢𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑛𝑜. 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑜𝑓 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 = 0. 318 %
𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑀𝑎𝑠𝑠 𝑜𝑓 𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝐶𝑜𝑝𝑝𝑒𝑟 (𝐼𝐼) 𝑆𝑢𝑙𝑓𝑎𝑡𝑒 = 0. 002 𝑔
% 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑀𝑎𝑠𝑠 𝑜𝑓 𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝐶𝑜𝑝𝑝𝑒𝑟 (𝐼𝐼) 𝑆𝑢𝑙𝑓𝑎𝑡𝑒 =
0.002𝑔
0.977 𝑔
× 100%
% 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑁𝑜. 𝑜𝑓 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝐴𝑛ℎ𝑦𝑑𝑟𝑜𝑢𝑠 𝐶𝑜𝑝𝑝𝑒𝑟 (𝐼𝐼) 𝑆𝑢𝑙𝑓𝑎𝑡𝑒 = 0. 205 %
% 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑁𝑜. 𝑜𝑓 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑜𝑓 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 1 𝑀𝑜𝑙𝑒 𝑜𝑓 𝐻𝑦𝑑
= 0. 205 % + 0. 318 %
= 0. 523 %
𝑜𝑙𝑢𝑡𝑒 𝑈𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 𝑜𝑓 𝑁𝑜. 𝑜𝑓 𝑀𝑜𝑙𝑒𝑠 𝑜𝑓 𝑊𝑎𝑡𝑒𝑟 𝑜𝑓 𝐶𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑧𝑎𝑡𝑖𝑜𝑛 𝑖𝑛 1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝐻𝑦
= 0. 523 % × 5. 7 𝑚𝑜𝑙
= 0. 029811 𝑚𝑜𝑙
≈ 0. 03 𝑚𝑜𝑙 (1 𝑠. 𝑓.)
*Rounding off of the final answer should be such that the absolute uncertainty should be first
rounded to 1 significant figure. Then, the number of moles should follow the number of
decimal places as the uncertainty.
𝐻𝑒𝑛𝑐𝑒,
𝑥 = 5. 70 ± 0. 03
1 𝑚𝑜𝑙 𝑜𝑓 𝐻𝑦𝑑𝑟𝑎𝑡𝑒𝑑 𝐶𝑢𝑆𝑂4 𝑦𝑖𝑒𝑙𝑑𝑠 (5. 70 ± 0. 03) 𝑚𝑜𝑙 𝑜𝑓 𝑤𝑎𝑡𝑒𝑟.
Calculations
Table 1. Raw data for the Mass of empty Crucible, Mass of Crucible & Copper(II) Sulfate
before heating, and Mass of Crucible & Copper(II) Sulfate after heating.
Trial
No.
Mass of
Crucible
(± 0. 001 𝑔)
Mass of Crucible + Copper (II)
Sulfate before heating
(± 0. 001 𝑔)
Mass of Crucible + Copper (II)
Sulfate after heating
(± 0. 001 𝑔)
1
32.009
33.614
32.986
2
22.833
23.834
23.462
3
33.930
34.346
34.189
4
33.570
34.568
34.050
7
33.125
34.140
33.755
Table 2. Processed Data for the Mass of Water, Moles of Water, and Percentage Uncertainties
of both.
Mass of water of
crystallisation
Trial
% uncertainty of mass of Moles of water of % uncertainty of moles
No.
water of crystallisation
crystallisation
of water of crystallisation
(± 0. 002 𝑔)
1
0.628
0.32
0.0349
0.3
2
0.372
0.54
0.0207
0.54
3
0.157
1.27
0.0087
1.27
4
0.518
0.25
0.0287
0.25
7
0.385
0.52
0.0214
0.52
Table 3. Processed Data for the Mass of Anhydrous Copper (II) Sulfate, Moles of Anhydrous
Copper (II) Sulfate, and Percentage Uncertainty of both.
Mass of Anhydrous
% uncertainty of moles
Trial Copper (II) Sulfate % uncertainty of mass of Moles of Anhydrous
of Anhydrous Copper
No.
Copper (II) Sulfate
Copper (II) Sulfate
(± 0. 002 𝑔)
(II) Sulfate
1
0.977
0.21
0.00612
0.21
2
0.629
0.32
0.00394
0.3
Mass of Anhydrous
% uncertainty of moles
Trial Copper (II) Sulfate % uncertainty of mass of Moles of Anhydrous
of Anhydrous Copper
No.
Copper (II) Sulfate
Copper (II) Sulfate
(± 0. 002 𝑔)
(II) Sulfate
3
0.259
0.77
0.00162
0.77
4
0.480
0.42
0.00301
0.48
5
0.630
0.32
0.00395
0.32
Table 4. Processed Data for the Mass of Anhydrous Copper (II) Sulfate, Moles of Anhydrous
Copper (II) Sulfate, and Percentage Uncertainty of both.
Moles of Water of
Trial
Crystallization in one mole of
% uncertainty
Absolute Uncertainty
No.
hydrated Copper(II) Sulfate
1
5.70
0.51
0.03
2
5.25
0.86
0.03
3
5.37
2.04
0.1
4
5.24
0.71
0.04
5
5.43
0.84
0.05
Final Mean Values
Using our various number of trials, we can find out the mean value of x so that we can come
with a concrete conclusion and compare our final result to the expected value.
𝑥𝑚𝑒𝑎𝑛 =
5.70 + 5.25 + 5.37 + 5.24 + 5.43
5
= 5. 40 𝑚𝑜𝑙𝑠
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑢𝑛𝑐𝑒𝑟𝑡𝑎𝑖𝑛𝑡𝑦 =
0.03 + 0.03 + 0.1 + 0.04 + 0.05
5
= 0. 25 𝑚𝑜𝑙 ≈ 0. 3 𝑚𝑜𝑙
𝐻𝑒𝑛𝑐𝑒,
𝑥 = 5. 4 ± 0. 3
𝑇ℎ𝑒𝑟𝑒 𝑎𝑟𝑒 (5. 4 ± 0. 3) 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑠 𝑦𝑖𝑒𝑙𝑑𝑒𝑑 𝑓𝑟𝑜𝑚 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 1 𝑚𝑜𝑙𝑒 𝑜𝑓 𝑐𝑜𝑝𝑝𝑒𝑟 𝑝𝑒𝑛𝑡𝑎ℎ𝑦𝑑𝑟𝑎𝑡𝑒.
Percentage Error
Percentage Error can be found using two ways. Percentage error and percentage uncertainty
can be easily differentiated. Percentage error is affected by the deviations of the instrument
measurements while error comes from the data.
𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝐸𝑟𝑟𝑜𝑟 =
±
𝑟𝑎𝑛𝑔𝑒
2
𝑅𝑎𝑛𝑔𝑒𝑥 = 5. 70 − 5. 24 = 0. 46 𝑚𝑜𝑙
𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝐸𝑟𝑟𝑜𝑟 =
0.46
=
2
0.23 𝑚𝑜𝑙
= 5.4 𝑚𝑜𝑙
±
% 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝐸𝑟𝑟𝑜𝑟
± 0. 23 𝑚𝑜𝑙
× 100% = 4. 26%
The above method is a useful way to find the percentage error if there is no expected value or
literature value to reference the observed value with. It is also useful for value sets with a
clear range. In our experiment, there is both a range since we repeated the experiment and
also a literature value for us to compare the observed values. The absolute error is a very
optimal primary method to solve for percentage error as it takes into account for other
random errors.
However, there are some limitations to using such a method to find the percentage error as it
does not consider the relation to the spread of the observed value from the expected value.
Method 1 considers the spread of the observed value from exactly half of the range. Hence,
there is also another way to calculate the percentage error:
𝐸𝑥𝑝𝑒𝑟𝑖𝑚𝑒𝑛𝑡𝑎𝑙 𝑉𝑎𝑙𝑢𝑒 − 𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒 |
% 𝑒𝑟𝑟𝑜𝑟 = ||
| × 100%
𝐸𝑥𝑝𝑒𝑐𝑡𝑒𝑑 𝑉𝑎𝑙𝑢𝑒
5.4−5
% 𝑒𝑟𝑟𝑜𝑟 = || 5 || × 100% =
0.4
5
× 100% = 8. 0%
This means that the results are very inaccurate and are collectively quite spread away from
the expected value.
Quantitative Data Analysis
If the percentage error is higher than the percent uncertainty, then random errors are
predominantly taking place. From the data, the mean percent uncertainty is 0.99% while the
percentage error is 8.0%. The percentage uncertainty is lower than the percentage error and
hence, it can be concluded that the errors do not take place within the theoretical limits of the
experimental errors. Furthermore, it can be concluded that random errors are extensively
taking place in the experiment.
Using prior knowledge, it is true that generally systematic errors affect the accuracy of the
data while the precision of the data set is unaffected. However, for random errors, both
accuracy and precision are affected.
For all the trials in the experiment, there are variations in their uncertainties. The lower the
initial mass of water reading, the higher the absolute uncertainty which will eventually lead to
a higher percentage uncertainty for the final number of moles of the water. Hence, analyzing
all of the data, we can come up with conclusions that suit the course of the investigation. For
example, for trial 1, the most amount of copper pentahydrate was added (1.605 g). This
resulted in the lowest percentage uncertainty for the number of moles of water. Furthermore,
trial 3, had the least amount of copper pentahydrate being added which resulted in the highest
overall percentage uncertainty. This clearly identifies the weaknesses of the entire
investigation and how the measurement uncertainty affects the interpretation and the analysis.
From the reasoning, it is clear that using more mass of copper pentahydrate would reduce the
overall percentage uncertainty. However, if so, then the percentage error would be greatly
scaled up in terms of its relation to percentage uncertainty. Hence, that will point out that
more random errors are bound to happen and occur, affecting both precision and accuracy.
Hence, there is a mixed argument on whether the amount of copper pentahydrate being used
is important.
Evaluating the reasoning behind both ways with respect to the aim of any experiment, it is
more important for the experimental errors to be within the theoretical error boundaries.
Although using more copper pentahydrate results, in a lower percentage uncertainty, using
less hydrous salt results in a high percentage uncertainty. With a higher percentage
uncertainty, there is a higher probability that the percentage error is lower than the %
uncertainty and a higher probability that the errors are within the theoretical range. Only then
will the experiment will be more accurate, not plagued by too many random errors.
Evaluation
Sources of error and Improvements
Sources of error affect the percentage error of the experiment. The first source of error would
be from the fact that copper (II) sulfate is hygroscopic and would be able to absorb water
vapor from the surroundings. Even after all the water is removed from the hydrous copper
sulfate, some water may be absorbed in the cooling phase, which will result in a higher mass
of anhydrous copper (II) sulfate that will affect the results of the experiment. According to
literature knowledge, I know that hygroscopicity is negligible under extreme heat. In order to
mitigate this, in the cooling phase of the crucible, the lid must be fully closed.
The second source of error is regarding human judgment in determining whether the
substance in the crucible is completely dry. Using observations also yields imperfections as
discussed in the Qualitative Analysis section before. It results in a lesser than supposed value
of the mass of anhydrous copper sulfate as some sulfur trioxide will escape from the crucible.
In order to mitigate this source of error, there should be constant observations of the crucible
and immediately turn the bunsen burner off once there is any release of white fumes.
However, this is caused by variations in human judgment and hence, it is inevitable.
Possible Extension
One possible and feasible way of extending this investigation is by comparing the degree of
hydration of copper (II) sulfate and other salts to derive relationships between them.