My ranger has gotten his alchemy skill up to 15 and can mix up a heavy heal potion from a medium and a light.

I know that practice makes perfect -- and the more of these he does, the better he gets<G>. (not to mention the fact that my Val gains from using her identify spell on the result) (not to mention the fact that the end result sells for more than the inputs cost).

My question is:

Does he get as much experience from mixing a stack of 5 each ingredients as he does from splitting the stacks and doing each one separate?

He gets more for mixing one by one. In fact 5 times as much. ;)