I'm taking the PCAT next week (AND I AM FREAKING OUT), but based on what I've seen about it, I wouldn't worry about memorizing too many formulas. Maybe some basic ones like, for gen chem, that G=H-TS or q=mcT.
I do not believe we need to memorize any masses, but you should know the basics like carbon and oxygen just from doing so many stupid mass problems. That being said, we also don't need to memorize the entire periodic table, but you should know the basic trends like electronegativity and size.
Lastly, for organic chemistry...who knows. Don't memorize mechanisms (thank goodness) - I've never seen a question about that.