Can i use my old MacBook charger on my new Mac?
The way Apple power adapters are designed is that you can use any charger on any laptop that has the same, or lower, power rating. For example, you can safely use an 85W power adapter (for MacBook Pro) on a MacBook Air (which requires only 45W) or MacBook (which requires 65W).

However this does not work the other way around, you cannot use a MacBook Air (45W) adapter to power a MacBook Pro.

A 2008 MacBook Pro adapter rated at 85W should power a 2011 MacBook Pro with no problem. If this is not the case, then the adapter is faulty.