If you're going to store your passport (or anything) in a zip-lock bag for a long period, maybe just sprinkle in some baking soda? Mould doesn't like dry, alkaline conditions.
Those little packs are completely useless if they've been exposed to open air for a few hours. I swear, 99% of the population think those packets reverse the laws of thermodynamics somehow, pumping an infinite amount of water without work.
I'll go with the chemically-valid, physics-valid, non-random white powder.
(I’m not seriously suggesting this)