I found there to be five waters of hydration in copper (II) sulphate hydrate. This is in agreement with the literature formula for hydrated copper sulphate of CuSO4∙5H2O. The ratio of moles remained constant because of the stoichiometric relationship between the water and the copper (II) sulphate, suggesting a chemical bond. The small variations between trials may be caused by improperly calibrated balances or not completely driving all the water off of the sample. I did not allow the sample to cool completely in the last run, so that might have introduced an error as well.
Evaluation of the Experiment (Skill E)
I did not encounter any anomalous results in my experiments, however, in experiment 2, a titre of 23.90 cm3 was obtained compared to 23.80 cm3 and 23.85cm3. These two results are concordant. As I was unsure if this was an anomalous result, I did the titration one more time and derived 23.90cm3 . From this, I can safely say that the titre volume of 23.80 cm3 may have been the slight anomalous result.
For experiment 1, I derived: 2.40g > 1.60g > 1.59g > 1.58g > 1.57g>1.57g.
A constant mass was achieved.
The results obtained for Copper were relatively different until a constant mass was achieved. This may have been because the Copper must have been heated for a little more than five minutes, which meant that some of the particles might have decomposed because overheating, can lead to decomposition of samples.
In addition to this, when reweighing the crucible, there may have been differences in the temperature (some crucibles may have been hotter than others), which could have caused a difference in the mass because hot objects expand.
Also, a high temperature will cause air currents around the balance, thus increasing the mass of copper.
In terms of the sources of errors for experiment 2, the conical flask may not have been washed properly which means that there may have been residues in the flask. This would make the titration inaccurate, as there would be remains of the previous titration in the flask. To overcome this, using distilled water to clean the flask after each titration would improve the accuracy.
Also, using pipette filler, air bubbles may have been formed at the tip as the solution was being drawn. This would have affected the quantity of solution being titrated, as it would have appeared to be less. If air bubbles do occur, then the solution should be released back and the process should be started again.
In terms of procedural errors for Experiment 2, if the reading is not taken from the bottom of the meniscus, then an inaccurate reading is taken, so this would lead to an error being created in the procedure. Also, in terms of the end-point of the experiment, it is every easy to over-titration and this may have been the case, so end-point should be approached with care and the 30 seconds wait should be kept to make sure that the colour change is permanent.
In addition, for the procedural error of Experiment 1, it was not repeated more than once so therefore, an average could not be found which mean that the one experiment that was conducted had to be taken as the true final mass. Also, the weighing scale measures to a degree of two decimal place; if it measured to four, this may have made the experiment more accurate.
In addition to this, the relative quantity used for Experiment 1 was a small mass of Copper which would have incurred a high percentage error.
For both experiments, the final results derived were almost accurate as the results obtained for experiment 1 were decreasing slightly and for experiment 2, they were concordant to each other.
In terms of accuracy and reliability of the experiment 2, a burette was used which has an accuracy of 0.05 cm3 . Using a burette, it is possible to control the experiment so that the colour change can be spotted easily. With the burette tap, drop by drop of Potassium Manganate can be added until a permanent colour change is noticed.
In terms of the technique used (titration), it was very accurate. It is used to find out how much of a chemical substance is dissolved in a particular solution (concentration) and this technique uses a particular set of apparatus with which volumes can be measured to an accuracy greater than 0.1cm3 and in this case, the apparatus used are, pipette, burette and volumetric flask. The pipette is used to give an accurate volume of solution which is usually 25.00 cm3 ; the burette is used to measure the volume of solution added and it has a degree of 0.05cm3 and the flask is used to make up an accurate volume of solution usually in a 250cm3.
However, with the burette, the reading should be always recorded at two decimal places and the second decimal place is an estimate so the uncertainty of the burette is +/-0.01ml.
Moreover, the colour change was observed very carefully. This is also known as the “end point”. A 30 seconds gap had to be given to the titration to determine if there was a permanent colour change. This improved the reliability of the results.
However, with experiment 1, it may not have been one hundred percent reliable because the heat from the Bunsen burner was not in a closed system, therefore, as the crucible is heated, some heat is also lost to the surroundings so therefore, the crystals are not being properly heated. Also, the crucible was not covered so therefore, there was heat loss, which could have affected the mass of hydrated copper crystals
Not withstanding this, the experiment can be said to be reliable as it was repeated many times to derive close-enough results.
Percentage error for 1st Experiment:
Weighing scale- degree of accuracy = 0.02
0.02 / 0.83 x 100 = 2.4%
A balance that measures to four decimal places should have been used to get a more accurate answer. This is the reason why the percentage error is high.
The percentage error should be less than 0.5%.
Percentage error for 2nd Experiment:
Burette- degree of accuracy = 0.05
0.05 / 23.86cm3 x 100 = 0.21%.
Pipette- degree of accuracy = 0.05
0.05 / 25cm3 x 100 = 0.20%
0.21% + 0.20% = 0.41%
The percentage error for the titration experiment is 0.41% altogether.
I am confident that the formula is right.
Also, from literature research, I found that the formula of hydrated copper crystals is CuSO4 5H20 (s).
This shows that the titration method is more accurate.
The mole ratios remained the same because of the relationship between the water and copper, however, there were small variations in the trials which could have been caused by the lack of drying of the Copper or improper calibrated weighing scales.
Looking at the method critically, with titration, over-titration has to be avoided. The colour change observed at the end is a little requirement but this has to be performed carefully so that it does not affect the result. In addition, the stir motor should not be allowed to run too fast because splashing may occur therefore, the reagents will not mix in. Whilst saying this, the burette should be centered over the flask so that drops fall directly into the solution otherwise, drops may fall to the side of the flask affecting the volume, thus, the reliability.
A main improvement that could be implemented had the equipment been available would be to completely automate the experiment to remove the degree of human error.
This would involve using very accurate robotic machines to precisely measure the amount of copper sulphate used and the mass of everything. It would also heat it for the exact amount of time to prevent burning (oxidization) or to not be fully dehydrated.
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