Serpentine Belt Runs On This Part... Help me find out what part?

[flipjarg]

Hi, I own a 99' Dodge Stratus. The engine is a 2.4. Lately i've been having problems with, what i've been told, the air conditioner. However i do not want to go purchase any parts unless i am possitive what this part is.

The serpentine belt runs on it. It is directly below the alternater. It's the lowest part running on the serpentine belt and closest to the front. Is there by any chance a picture or diagram i can look at that will give me the name of the part?

Once i figure this out i get to put on a new serpentine belt. Just curious have other people had an extremely hard time putting the serpentine belt on your 99 Dodge Stratus? i've done it once since i've owned the car and it was hell.