To determine the molecular formula of hydrated copper sulfate.
Hydrous copper sulfate
First I weighed 3g of hydrous copper sulfate on electronic balance and began heating it in an evaporating dish on top of a Bunsen burner. After the reaction was complete I measured the new weight to find a change in mass.
Qualitative data –
After the reaction was complete, the colour changed from blue to white due to a loss of water crystals.
Faint green tinge became evident and fine white powder was formed from crystals. Continued heating led to formation of oliver green colour.
Quantitative data –
Original weight of hydrous copper sulfate – 3g
Error in electronic balance – ± 0.001g (provided by the teacher)
Data collection –
Data Processing –
To minimize the errors I conducted three trials and averaged the readings to find a more accurate answer.
= 1.93 g ± 0.001
Change in mass = (3 ± 0.001) – (1.93 ± 0.001)
= 1.07 ± 0.002
The change in mass was due to the loss of water crystals. Water (H20) has a molar mass of 18g. Therefore the change in mass must be equal to the mass of water present in copper sulfate.
No of moles of water evaporated = = 0.059 ± 0.002moles
The molar mass of anhydrous copper sulfate is 159.6g.
No of moles of = = 0.012 ± 0.001moles
In the equation I took the no of moles of water as x because it is unknown.
CuSO4.xH20 → CuSO4 + xH2O
According to the equation the mole ratio between copper sulfate and water is 1: x. Therefore 1:x = 0.012 ± 0.001: 0.059 ± 0.002
x = = 4.9 ± 0.588
Hence the x value can be rounded off to 5 to get an integer answer. This means that in 1 molecule of hydrous copper sulfate there are 5 atoms of water present as crystals. The result is also equal to the literature value which was provided by the teacher and there seems to be no deviation because the answers are taken as integer and decimal points are excluded. The only systematic error present was in electronic balance. Moreover random error could have been caused during heating of the salt i.e. overheating or underheating. This error depends on the sight of colour change, therefore this is a very hard random error to eliminate.
As the answer is taken as integer, there seems to be zero error however still a lot of errors are possible while conducting this experiment. During the heating of hydrous copper sulfate there maybe some water crystals still left due to inappropriate stirring and overheated or underheated salts could have also caused error. Main error which is inevitable is the hydration of crystals after being heated. The reaction is reversible and the after heating there could have been water vapour in the air which could have combined with the salt. Hence these can also lead to deviations in results. To improve upon these errors we must make sure that after the salt is heated, its weight must be measured immediately so that there is very less time gap between it. Furthermore proper stirring and care taken during heating can also minimize errors.