What Wednesday said. They just want to see where you are science-wise, and since General Chem is a pre-req for Orgo and Bio at Columbia, they want to start you off on the right foot. That way, if you need extra help with Chemistry they'll stick you in a pre-chem class right away so you can relearn what's been forgotten. I took the test and got an 18; because I was so close to the 20s they said I could retake it a little later to try and place into Chem I, but I know my weaknesses and asked to take pre-chem anyway. I feel it's better to start off with the basics rather than jump in headfirst and try not to drown.
At any rate, Columbia seems to take their math placement exam more seriously than their chem placement exam--that's the impression I got, anyway--so I wouldn't stress. Just get a Chem book (or, as Wednesday said, AP Chem) and go over it; or, if you really don't feel all that confident with Chemistry, go ahead and take the pre-chem course anyway. They're offering it in the summer, which is when I'm taking it, so I can get ready for Chem I and Physics I next Fall. An extra year is worth the prep time.