by slparker » Tue 05 Jul, 2016 9:59 am
You're looking at simple concentration expressed as a percentage of weight per volume.
Pharmaceutical solutions are expressed as weight per volume i.e. a 0.1% adrenaline solution is 1mg/ml (or 1g/L). You don't need the molecular weight for this calculation - it is an expression of weight of solute to solvent in grams/L.
The OFF! Tropical strength at 191g/L is a 19.1% solution. A 20% solution ought to be 200g/L. If you buy the Off tropical strength you will not achieve the 20% the OP is after but it is pretty close. That wouldn't worry me at all. I would just wear long sleeves, soak my clothes in permethrin and spray my exposed skin with the 19.1% solution.
Once it is prepared in the factory the molecular weight, temperature, latitude or angle of your tongue does not effect the concentration of the solution. I am not sure of the stability of the picaridin compound though - so long as you use it before the use by date it should be fine.