1) Generally, if something breaks up into ions in a solution, the number of each atom in the compound shows how many it breaks up into.
Eg - Ca(OH)2 -> Ca2+ + 2OH-
2) You just need to know which combination of ions precipitate, there are many solubility charts online. Here's one:
Solubility chart - Wikipedia, the free encyclopedia
3) Cu(II) tells you that its a copper molecule with a charge of 2+
Similarly if it was Cu (I) it would only have +1 charge.
Since you know (or should know) that Cl has a charge of -1, it would take 2 Cl molecules to bond with one Cu (II)
Hence: CuCl2
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