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"F" isn't a unit of concentration I recognize, are you sure that's correct? Is there any more information you can give?
i think f stands for formality.
Formality is an old term, rarely used anymore, from what I've read. See below: "The purpose of formality is to distinguish the number of moles of a compound from the number of moles of ions in solutions of ionic compounds or weak electrolytes. If we dissolve one mole of calcium nitrate in enough water to make a liter of solution, the formality is one. The molarity (as it is commonly used) of calcium nitrate is the same as the formality, but that is a little sloppy because, once dissolved, the calcium nitrate ionizes completely so there isn’t really any Ca(NO3)2 in the solution. The molarity of the nitrate ions is two molar, because one formula unit has two nitrate ions. The formality of nitrate ion doesn’t exist because it is defined as the number of moles of the entire compound. So the only difference between formality and molarity is that you can express the molarity of the different ions individually and the formality is the entire compound irrespective of ionization" from http://www.chemprofessor.com/conc.htm