What is formality?

1 Answer
Oct 8, 2015

Here's how you can think about formality.


Simply put, formality is a way of expressing the concentration of a compound in solution.

More specifically, formality is defined as moles of solute per liters of solution.

This should sound very familiar - isn't molarity defined as moles of solute per liters of solution?

So is fomality the same as molarity? Sometimes.

Formality does not take into account what happens to a chemical compound once it's dissolved. That is, it won't matter if the compound is an electrolyte or a a non-electrolyte.

If you dissolve sodium chloride, #"NaCl"#, in water, you know that it dissociates completely and exists as ions in solution

#"NaCl"_text((aq]) -> "Na"_text((aq])^(+) + "Cl"_text((aq])^(-)#

So if you dissolve one mole of #"NaCl"# in one liter of water, you will have

#F = "moles of solute"/"liters of solution"#

#F = "1 mole NaCl"/"1 L" = "1 formal"#

The molarity of this solution, on the other hand, is said to be #"1 molar"# in sodium cations and #"1 molar"# in chloride anions, not #"1 molar"# in sodium chloride.

And here's the difference between formality and molarity.

Molarity actually takes into account what happens to the solute once it's dissolved. Formality does not.

Check out these examples for a more detailed explanation