Dear Micol Caichiolo,

The following info-sources below may help:

4.5. How to prepare a copper (II) sulfate solution?

https://www.periodni.com/enig/solcalc_help/preparing_copper_sulfate.html

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https://www.researchgate.net/topic/Copper-Sulfate

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How many grams of CuSO4⋅5H2O are needed to prepare 100 mL of a 0.10 M solution

https://socratic.org/questions/how-many-grams-of-cuso-45h-2o-are-needed-to-prepare-100-ml-of-a-0-10-m-solution

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Copper sulfate pentahydrate:

https://pubchem.ncbi.nlm.nih.gov/compound/Copper-sulfate-pentahydrate

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First of all, you need to avoid moisture from the air during such work. You can use a dry room with air conditioning or a glove box.

If you want to prepare an aqueous solution of CuSO4, it doesn't matter what type of crystals you will use. You just need to take into account the hydration water contained in the crystals. If the crystals are blue, then you have CuSO4 * 5H2O. When their colour is going towards white, they are less hydrated. Anhydrous CuSO4 is white (but is easily confused it with for example, white-celadon color powder).

If you want to be sure about the composition of solution, I would use pentahydrate CuSO4 and calculate how much water you need to use. See the answer by Len Leonid Mizrah.

Perhaps of some interest for your query:

https://www.researchgate.net/post/What-is-the-best-experimental-procedure-to-determine-the-solubility-of-copper-sulphate-at-different-temperatures