posted by Isaac
When is the term, formula unit, useful?
Is it when trying to figure out the empirical formula?
Formula unit usually is used in connection with the "molar mass" of the compound. Some profs don't like to use the molar mass term because in some compounds, especially ionic compounds, the term mol is misleading. For example, we know that the substance CO2 occurs as discrete molecules so a mole of CO2 has a molar mass of 44. But in a compound like NaCl, that is an array of Na^+ and Cl^- that occur in a three dimensional array (lattice if you will) of the ions. We write the formula as NaCl because there is a 1:1 ratio of the ions; however, the actual formula should be written as NaxClx. The point of this is that there may be 100 Na ions and 100 Cl ions or 10,000 of each. There is always a 1:1 ratio but just how large is the array we are discussing. That of course varies with how much of the crystal we cut off to weigh or measure. So to get around the fact that NaCl molecules are not really discrete molecules, the profs prefer to use the term formula unit mass. In practice the formula unit mass is synonymous with molar mass but they are not the same thing although the value of the number we use is the same. The formula unit prof will tell you there is no such thing as a molecule of NaCl (and they are right) so there can't be a molar mass but there is a formula mass since we define formula unit as NaCl.
So it's used to create a mass for a molecule that doesn't exist, if what I am getting is correct. So by using the ratio between the two elements, especially those in an ionic compound such as NaCl, it's there to give a value to something that never had a value.
I think your wording, although not exactly incorrect, implies that the term formula unit is a "fix" for a problem that doesn't exist and that isn't quite true. It is a "fix" for a problem that does exist. The idea is "don't call it a molar mass of NaCl" since there is no such thing; rather, call it what it is and that is formula unit.