PROCEDURE:
- Take a crucible and weigh it on the electronic balance.
- Then carefully fill the crucible with 2.5 grams of hydrated copper sulphate.
- Carefully light the Bunsen burner using the spirit lamp and leave the air hole on the barrel halfway open in order to provide a non-luminous flame.
- Place the crucible on a stand supported by insulating material and heat it with the flame of the Bunsen burner. The crucible should have the lid over it while heating.
- While heating lift up the lid using the tongs at regular intervals to check if the copper sulphate has turned completely white.
- When copper sulphate becomes a white anhydrous powder it means that it has lost its water of crystallization. At that time stop heating and let the crucible cool.
- Once the crucible containing white copper sulphate has cooled retake the mass the mass of crucible containing white copper sulphate and note the difference in mass.
- The loss in mass is accounted for due to loss of water of crystallization in the form of water vapour.
- Then the no. Of molecules of water of crystallization can be calculated by co relating the mass with the Relative Molecular Mass.
- Repeat the procedure about five times in order to get a high level of accuracy.
OBSERVATIONS AND READINGS:
The following observations were made during the experiment:
- The colour of copper sulphate changed from blue to white.
- The hydrated salt changed into a white amorphous powder due to a loss in water of crystallization.
- There is consequently a loss in weight in copper sulphate.
Mass of I mole of hydrated copper sulphate = 1Cu = 63.55
1S = 32.06
4O = 64
10H = 10.1
5O = 80
Therefore the RMM = 249.71
Hence no. Of moles in 2.5 grams = (2.5)(1)/249.71 >> 0.01mole.
Mass of hydrated copper sulphate taken: 2.5 grams.
TABLE 1 (Showing change in the mass)
Mean loss in mass= 0.83+0.84+0.85+0.84+0.86/5 = 0.884 grams.
Mean mass after heating = 1.656 grams
Mass of 1 molecule of water (0.01 mole) = 0.1802 grams
Therefore no. Of water molecules in copper sulphate = 0.884/0.1802 = 4.9
Hence it can be assumed that there are 5 molecules of water of crystallization in hydrated copper sulphate.
ERROR ANALYSIS:
% Error in the electronic balance = +-(0.01)(100)/2.5 = +- 0.4%
%Error in the electronic balance = +-(0.01)(100)/1.656 = +-0.6%
Total systematic error= +-1%.
ERROR EVALUATION:
- The total systematic error in the experiment accounted to 1%.
- The rest of the inaccuracy can be accounted for by the random errors.
- The mass of anhydrous copper sulphate could have increased as few of the water droplets could have trickled back into the crucible.
- It is quite likely that heating was ceased before the hydrated copper sulphate changed to a completely white anhydrous powder.
- The fluctuating conditions of temperature and moisture could have altered the results.
- One cannot assume that the samples used were 100% pure.
- A small portion of the sample could have been spilled while transferring it.
CONCLUSION:
- The number of molecules of water of crystallization in hydrated copper sulphate is five.
- The mean loss in mass was calculated to 0.884 grams.
- The loss in mass is accounted for by the evaporation of water vapor while heating.
- This evaporated water vapor was initially stored as molecules of water of crystallization in loose chemical combination with the compound.
- The reason why the hydrated salt changes to white amorphous powder is that there is loss in the water of crystallization and this property is known as efflorescence.
- The results obtained are fairly accurate and precise as the value is extremely close to the literature value. (4.9 molecules as compared to 5).
- The systematic error in the experiment accounts to +-1%.
SUGGESTED IMPROVEMENTS:
The following steps if followed can help to increase the accuracy of the experiment:
- The number of trials can be increased.
- The samples used must be 100% pure.
- Random errors must be avoided as far as possible.
- Care must be taken to ensure that the sample turns completely white on heating and no water droplets fall back into it.