When to use “which” and when to use “that” hound me.  And maybe you too.  Here is the rule.  Good luck.
The rule of thumb, then, is that which clauses are nonrestrictive (nonessential)while that clauses are restrictive (essential). Nonrestrictive clauses and phrases are set off from the rest of a sentence by a pair of commas (as in our examples above) or by a single comma if they come at the end of the sentence. (Example: “I took a vacation day on my birthday, which happened to fall on a Monday this year.”)
Merriam-Webster’s Collegiate Dictionary (10th edition), regarded by most writers as the authority on such matters, tells us that it is now common for which to be used with either kind of clause, while that must be used only for restrictive clauses. In fact, though, careful writers continue to make the distinction we describe above. Attorneys are taught to use which for nonrestrictive clauses and that for restrictive clauses so as not to cause a misreading in legal documents. It seems just as important that we work to avoid misreadings in all writing, not only in situations when a legal ruling might be at stake.