Water of crystallization answer

A watch glass was weighed and then a thin layer of hydrated copper(II) sulfate was carefully added. The watch glass and salt was now reweighed and placed in an oven at 250 ºC for two hours. The watch glass was then removed and allowed to cool in a desiccator before a final weighing. Use the data below to calculate the number of moles of water of crystallization in the original salt. Suggest why the experiment might give an incorrect answer when the oven was set at 150 ºC and if the salt was heated very strongly in a crucible. Why was a watch glass chosen rather than a crucible? Why was around 8 grams of salt chosen? How could the experiment be improved?

Mass of watch glass = 25.233 g

Mass of watch glass and hydrated salt = 33.789 g

Mass of watch glass and salt after heating = 30.726 g

Mass of anhydrous salt = 30.726 - 25.233 = 5.493 g

CuSO₄ = 159.5

\number of moles of CuSO₄ = 5.493/159.5 = 0.0344 mol

Mass of water lost = 33.789 - 30.726 = 3.063 g

H₂O = 18

\number of moles of H₂O = 3.063/18 = 0.170 mol

Number of water moles per mole of salt = 0.170/0.0344 = 4.95

\formula of salt = CuSO₄.5H₂O

At the lower temperature there appears to be less water of crystallization as not all of the water is driven off. At the higher temperature there appears to be more water of crystallization as some thermal decomposition of the CuSO₄ takes place.

We chose a watch glass as it allowed the salt to be spread out more thinly than would be possible in a small crucible. An open glass petri dish could also have been used. With a large bulk of solid, the water from the centre of the bulk may not have been readily lost.

A very small mass of solid would have more easily lost its water, but this would have led to a large percentage weighing error. A very large mass of salt would have not easily lost water from the centre of the bulk. A quantity in the range of 1 to 10 grams is ideal.

For improvement we should have done at least two experiments so that we could get some concordant results. The instructions say to heat for two hours, but if we are planning our own experiment we need to decide how long the heating needs to be done for. The usual technique is to heat to constant mass. After the final weighing in the instructions, the sample is returned to the oven for another twenty minutes, cooled and reweighed. This process is repeated until there is no further mass loss.