Formal Concentration Units

The concentration unit, formal, is similar to the more familiar molar concentration in that it is calculated as the number of moles of a substance in a liter of solution. Formal concentrations are notated with the symbol F, and are formula weights per liter of solution. A formula weight is the weight of a mole of the substance. The difference between formal and molar is that the formal concentration indicates moles of the original chemical formula in solution, without regard for the species that actually exist in solution. Molar concentration, on the other hand, is the concentration of species in solution.

Let's look at an example of how how formal concentrations are calculated. We'll use calcium carbonate because it has a simple formula weight (100 g/mole). If one has 1 g CaCO3 in 1 L aqueous solution, the concentration of CaCO3, in formal, is...

1 g CaCO3/(100 g CaCO3/mole)/(1 L) = 0.01 F CaCO3

How is the formal concentration unit different than the usual molar (M in moles per liter) unit? Actually, they differ in a technical sense. The same formula weight based calculations are used. However, CaCO3 does not exist in solution to any great extent. In fact, this salt ionizes in acidic solution to form calcium ion, Ca2+, and the carbonate anion, CO32-. Carbonate is a base and, in turn, reacts with water to produce protonated forms; HCO3- and H2CO3. The solution is not 1 M CaCO3. In fact is is nearly 0 M CaCO3 because the calcium carbonate has ionized and reacted with water and H3O+!

In this case the formal concentration unit is used to account for the fact that there is .01 mole of CaCO3 in the flask, though it does not exist as such in this form. From this we know that there is 0.01 mole of calcium and 0.01 mole of CO32- in all of its various forms.

August 03, 2004