Determining the water of crystalisation

Drawing 1: The experiment

Observations:

Crystals of CuSO4*xH2O are blue due to water of hydration. When heated, the colour gradually changes from blue to gray-white. It's the effect of losing water molecules. The process can be easily observed on sides of crucible first. When the crystals are heated in temperature over 470 K, they lose all the water molecules.

        

Data Collection and Analysis:

Mass of crucible                                         29.58 ± 0.05 g

Mass of crucible and hydrated salt                          31.56 ± 0.05 g

Mass of crucible and salt after first heating                 30.85 ± 0.05 g

Mass of crucible and salt after second heating          30.84 ± 0.05 g

Using the molar mass of the anhydrous salt I used I was able to calculate the number of moles of anhydrous salt that remained after heating.

MCuSO4= 63.55+32.07+15.99*4=159.58 g

1 mole of CuSO4 -------- 159.58 g

X                        ---------- 1.26 g

X=(1 mole*1.26g)/159.58 g

X=0.007896 moles of CuSO4

Rounded to 6 decimal place

Then, the mass and number of moles of water that was lost from the salt were calculated.

31.56 g - 30.84 g = 0.72 g

MH20=15.99+2*1.01=18.01 g

1 mole of H2O --------  18.01 g

X          ---------  0.72 g

X= (1 mole*0.72g)/ 18.01 g

X=0.039978 moles of H2O

Rounded to 6 decimal place

After calculating both number of moles, I was able to calculate the number of moles of water that are combined with one mole of the salt.

Moles of CuSO4  :    Moles of H2O

0.007958     : 0.039978

                               1      :   0.039978/0.007896

              1      :    5.06307

  1      :    5

As the number of moles should be an integer, the formula of hydrated salt is CuSO4*5H2O

Conclusion:

I am satisfied with the results of my work, they are almost perfect as my calculations gave the result slightly bigger than the real one; errors were the cause.

In the experiment there was error due to uncertainties of measurements, a half of the smallest unit- 0.05 g. Moreover, the uncertainties came into existence due to the lack of the precision of the balance. It was impossible to be precise as the weighting machine was wobbling. Another error could occur when the crystals were chosen. It's better to take a smaller amount of crystals rather than bigger, so the reaction can be observed more easily. Additionally, the crystal should not be to small. Errors also occurred when re-weighting the crucible. It was important to cool the crucible before weighting. Not only was it for our safety, but also for the precision of our measurement as the weighting machine could be influenced by heat. So an error could occur. An error could come up while stirring. It is possible that part of our substance was left on the stick what cause further errors.

To improve the experiment

• larger portion of salt
• lid on the crucible (to avoid loss of crystals)
• wider crucible (easier to heat)
• more measurements
• more time to cool down the crucible
• weighting machine with more decimals (to lower the uncertainties)

could be used.

Also, there are some hydrates, which don't decompose before reaching their boiling point. For those, the method used in the experiment is not suitable. Removing water from melted form and crystalliation from anhydrous salt could be adopted.