new pit bull puppy

Any breed will 'try' you, its called adolescense (sp?) They are in between puppyhood and adulthood, and just like teenagers they want to push all your buttons. No pit bull should 'turn' on you, they were not bred to be human agressive, plain and simple.
 
Just as I recommend with any other breed, get this girl into training asap. That way she will learn her boundaries, and when/if she enters into her terrible doggy teens she wont try and push the limits.

Julitza is exactly right, 1.5 years is right around the age when all dogs of all breeds are entering their adulthood, and many undergo some personality changes. They could range from just being willful or stubborn, to showing aggression behaviors... but it's definitely a good time to brush up or perhaps begin on obedience training.
 
Some pit bulls can be dog aggressive (of course, some dogs of any breed can be dog aggressive, too) but the breed is not known for human aggression - these are two different behavior patterns in dogs. In fact pits are known for being great family dogs and good with kids. Anyone who tells you pit bulls attack and kill humans out of some kind of natural bloodlust is just plain wrong and misinformed, period.
 
Congrats on adopting your puppy!

This breed is not known for dog to human aggression.  However, with any dog, early socialization and training is key.  I am not encouraged that she was chained, but if she also had contact with dogs and humans, perhaps it will work out ok.  Run, do not walk, to the nearest facility that offers positive training and get her in to puppy class.  This is a breed that needs constant socialization until maturity.
After that, not a dog park kind of dog, most probably. 
These sites have trainer searches:
[linkhttp://www.peaceablepaws.com]www.peaceablepaws.com[/link]
[linkhttp://www.apdt.com]www.apdt.com[/link] (make sure no choke chains, etc., no matter what)
[linkhttp://www.clickertraining.com]www.clickertraining.com[/link]

Make sure the trainer you use has experience with the Bully breeds and is not biased against them, or naive about the possibility of dog to dog aggression later on.  Do not allow your son to roughhouse with the dog, or be on the floor at face level.  The dog should consider him a leader (it would be a great experience for him to handle the dog in class) and not a peer or playmate.  Dogs play rough (all dogs, not just Pitties).  
The cautions are for your protection, but should not put you off from owning this breed.  They simply require appropriate management.  I have a great couple who just graduated from class with a one year old Pit who walks nicely on leash, doesn't pull, comes when called, and has learned sit, stay, leave it, and down, as well.  This is a breed that you need to train early since they are so powerful and muscular.  You can do it!!!

I knew a VERY sweet male fixed pit bull that wouldn't hurt a fly, BUT HE LOVED TO PULL!!! He lied w/ two little girls and they where pretty nice but they could have enoyed one of my dogs! He was soo loyal until he started to jump the 3 ft fence and poeple stared to complain... He never hurt anyone though! HIS NAME WAS: Soldier

I appreciate all the replies from everyone--its makes me much more comfortable with my decision about keeping the puppy--I have heard from friends today that we do have obedience training in my area and I plan to take her very soon--they said her age is perfect to start--everyone that has seen her says she will be a big dog-lots different from the little dacshund I had--it will be a challenge but hopefully a very rewarding one-again, thanks for all you kind words

Oh, we named her Elly