<p>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.</p>
<p>Eg - Ca(OH)2 -> Ca2+ + 2OH-</p>
<p>2) You just need to know which combination of ions precipitate, there are many solubility charts online. Here’s one:</p>
<p>[Solubility</a> chart - Wikipedia, the free encyclopedia](<a href=“http://en.wikipedia.org/wiki/Solubility_chart]Solubility”>Solubility chart - Wikipedia)</p>
<p>3) Cu(II) tells you that its a copper molecule with a charge of 2+</p>
<p>Similarly if it was Cu (I) it would only have +1 charge.</p>
<p>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)</p>
<p>Hence: CuCl2</p>