If I rent a music cd from the library can I rip music from it onto my computer and burn a copy onto a blank cd?

Definitely illegal but how can any one really enforce that?

You would be violating the copywrite laws. The people who made the CD earn their living from its sales, so you would beat robbing them of their earnings.

Rent? You mean borrow? Idk if you can but technically its pirating and illegal.. you can just buy an old cd from ebay or watch them free on youtube