# Chemistry

When is the term, formula unit, useful?

Is it when trying to figure out the empirical formula?

1. 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.

posted by DrBob222
2. 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.

posted by Isaac
3. 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.

posted by DrBob222

## Similar Questions

1. ### chemistry - empirical formula

write the empirical formula for each of the following molecular formulae" a)P4O10 b)C2H6 Confused as to what the diff.is b/w empirical and formula...have no idea how to answer these b/c they look like empirical formulas to me
2. ### Chemistry

If you divide the molar mass of a compound by the empirical formula mass, what is the result? That division should be VERY close to a whole number. Usually, 0.9 to 1.1 or 1.9 to 2.1 (all depending upon the accuracy and precision
3. ### chemistry

How do I find the empirical formula for aluminium selenide? You have to have some data, such as masses, or percent: Empirical means it was measured. The empirical formula for aluminium selenide is the same as it's molecular
4. ### Chemistry

Please check my answers for the following questions. THANK YOU. 1) The empirical formula of a compound that is 25.9% nitrogen and 74.1% oxygen is.... ANSWER: N2O 2) empirical formula of a compound that consists of 4.80 grams of
5. ### chemistry

Determining a Molecular Formula from an Empirical Formula 1. A compund has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?
6. ### chemistry

Determining a Molecular Formula from an Empirical Formula 1. A compund has an experimental molar mass of 78g/mol. Its empirical formula is CH. What is its molecular formula?
7. ### chemistry

I need to know if I have th right molecular formula for Sorbito 39.56g C 7.75g H 52.7g O Empirical Formula is CH2O 30.026 amu how I how calculate the molecular mass and how I figure out the melecular formula? please help
8. ### Chemistry

the formula H2O2 is an example of 1.molecular formula 2. an empirical formula 3.an ionic formula 4.an organic formula
9. ### Chemistry (Check)

True or False The empirical formula of a compound is always the same as the molecular compound. My Answear: True The empirical formula for acetylene is CH. The molecular formula is C2H2. Another example: The empirical formula for
10. ### chemistry

why is the molecular formula of oide of arsenic in its gaseous state is not neccessarialy the same as its empirical formula what does this mean and how do i do it Molecules are known to dimerize or trimerize. We actually don't

More Similar Questions