nook, your final mark for internals is basically your position from the best mark in your grade in that course, scaled accordingly to how well your class(s) did.
The externals.. it will be fitted to a bellcurve depending on how well people from this course did on other subjects: thus determing this subjects "difficulty" I'm guessing an average mark of say, 70, will go up to about 75, but that's just a guess.
Your final mark for chem will be internal(scaled)+external(scaled)/2, pretty much.
Good luck btw everyone