Dehydration of Copper Sulfate

One of Robert Bruce Thompson’s Illustrated Guide to Home Chemistry Experiments was determining the formula of a hydrate. I used copper sulfate, as it is easy to see the difference between the normal pentahydrate state (bright blue) and the anhydrous state (white).

I weighed out 4.96 g of CuSO4 into a crucible and heated it in an old oven for about 23 minutes (probably don’t use an oven that is used to prepare food). After this time, there was still a slight blue tinge, so all the water of hydration probably wasn’t driven off. The net weight was 3.53 g, so there was a 1.43 g weight loss.

As the molar mass of copper sulfate pentahydrate is 249.7 g/mol, the amount of moles to start was: n = m/MM = 4.96 g/ 249.7 g/mol = 0.01986 moles.

As the molar mass of anhydrous copper sulfate is 159.6 g/mol, the amount of moles of anhydrous copper sulfate is: n = m/MM = 3.53 g/ 159.6 g/mol = 0.02212 moles.

As the calculations suggest there is more anhydrous copper sulfate than there was copper sulfate pentahydrate to start off with, it is very likely that not all the water of hydrate was driven off, so it is making it appear that there is more anhydrous copper sulfate than there really is.

The amount of water can also be calculated, as the molar mass of water is 18.015 g/mol:
n = m/MM = 1.43 g/ 18.015 g/mol = 0.07938 moles.

The proportion of water to copper sulfate pentahydrate (in molar equivalents) is
proportion = 0.07938 moles water / 0.01986 moles CuSO4 = 3.997

The calculations suggest there are 4 water molecules for every molecule of CuSO4, but there are known to be 5 water molecules of hydrate, not all the water was driven off. Perhaps a longer time in the oven, of around 40 minutes might work.

 

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