Chemistry Lab Report. AIM: To calculate the number of molecules of water of crystallization in hydrated copper sulphate.

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.