Howdy!
If it were me, I would only mix enough Ferric Chloride to etch the board that you have already setup, and save the other tabs for later PCB's that you'll make. Plus, if you mix all of the tabs, and accidentally mix in too much water, you''ll pretty much have to go down to the store and get some more Ferric Chloride tabs. Mixing a little at a time and making a mistake wont put you out a whole pack of tabs.
I use a tupperware container to store my Ferric Chloride. In the case that you may not be familiar with tupperware, tupperware is a plastic container that is used to store food, or other products in. The plastic container has a lid that fits tightly in order to keep most liquid from leaking out. I'm sure any plastic container will do okay, as long as the thickness of the container isn't very thin (Like those disposable plastic containers).
I keep my ferric chloride in a cabinet away from other chemicals such as bleach or amonia. If these two chemicals (Ferric Chloride, and amonia or bleach) were to mix, I'm sure the gasses that would be produced, would be pretty nasty, and possibly lethal.
Good luck!