If you used a beam balance, then, depending on its condition, it could be the cause of inaccurate weighings. Some of our old ones are rusty and the arm does not swing freely, causing some readings to be a little high and others low. But, if your balance was in good condition, and was capable of measuring to at least 0.01 g, then it is probably not to blame, unless you were weighing very small quantities like a few tenths of grams. But also, some students are just more patient and careful using the balances and get better weighings as a result. But that's a "human" element.
There probably are not any volatile imputites in the hydrated CuSO4, so the extra loss in weight that caused you to get a slightly high amount of water may be due to a loss in weight another way, such as spattering out or oozing out of the crucible during the heating. Did you stir the contents with a stirring rod to break up lumps? If so, some material likely stuck to the stirring rod and would be lost that way. Probably the sample itself was of good purity. If you were weighing very small amounts of compound, in the tenths of grams range, then inherent weighing uncertainty in any general purpose lab balance could cause an error of plus or minus an H2O in your calculated formula.
It's also possible that overheating could cause some decomposition of the CuSO4. This would cause an excessive loss of weight:
"The pentahydrate is 100% isolable only in temperatures lower than 30 degrees celcius. The light blue trihydrate non-isolable form can be obtained around 30°C. White monohydrate form is available at 110°C, while the anhydrous form can be isolated near 250°C. The decomposition slightly starts at 250 degrees, while complete decomposition occurs around 600 degrees...
The reaction is as follows: CuSO4 (aq) ==> SO2(g) + CuO(s)
If you smelled any SO2 fumes, that's what happened! In a hot Bunsen burner flame, you are getting near 600°C.
One more thing, on the "human" side again - be careful not to round numbers too much during calculations. I've seen students round off numbers like 1.14 g to 1.1 g, or worse, 1.42 g to 1 g. This really affects the calculation! This rounding may be justified "mathematically" speaking, but it's best to not round numbers during calculations, just round off the final answer at the end, to avoid accumulating "roundoff error". Use a formula weight of 159.609 g/mol in your calculation, not 160, etc., just to be on the safe side!
Steve
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