Identify the comma rule that the following sentence breaks:

Although she loved her parents Sarah was eager to leave for college and be independent.

Use a comma to set off a phrase or cause at the beginning of a sentence

The comma rule that the sentence breaks is the rule for using commas with coordinating conjunctions.

Study this site, and then tell us what you think.

http://grammar.ccc.commnet.edu/grammar/commas.htm

Use a comma and a conjunction to separate two complete thoughts.