Empirical & Molecular Formula

Empirical formula: is the lowest ratio of atoms/moles in a formula.

Note: All ionic compounds are in its empirical formula

An example would be...

H2F10 is a molecular formula which reduces into HF5 which is the empirical formula!

For example, we were given  58.5% of carbon, 7.3% of hydrogen, and 34.1% nitrogen. What is the empirical formula?

* Assume that there's 100.0g*

1) Convert grams --> moles

C: 58.5 g x 1 mol = 4.88 mol               
                     12.0 g
H: 7.3 g x 1 mol  = 7.3 mol
                   1.0 g
N: 34.1 g x 1 mol= 2.44 mol
                      14.0g
2) Divide all 3 by smallest molar amount.
C: 4.88/2.44= 2
H: 7.3/2.44= 2.99 --> 3 (Round it only if it's super close!)
N: 2.44/2.44= 1
Empirical Formula= C2H3N
3) Scale ratios to whole numbers if needed. (For this example, it isn't needed..but if for example you ended up with 1.5, multiple it by 2,3,4,5, etc until you get a whole number.)

Here's a nice little flow chart to show you the steps:

 

Molecular Formula (MF): multiples of an empirical formula & shows the actual number of atoms that combine to form a molecule.
To calculate the MF: 
                         n=      molar mass of compound       
                               molar mass of empirical formula 
 n= a whole number multiple of the empirical mass
So then... MF= N x empirical formula
Ex. A molecule has an empirical formula of HO and a molar mass 51.0 g. What's the molecular formula?
Mass of HO= 17.0 g
51.0 g= 3
17.0g                               
Using the above formula of MF=N x empirical formula --> 3x(HO)= H303
Ex. A compound contains 8.96 g of Nitrogen and 5.19 g of Oxygen. The molar mass of the compound is 88.0 g. What's the molecular mass?
N: 8.96g x 1 mol = 0.64 mol

                    14.0g

O: 5.19g x 1 mol  =0.32 mol
                    16.0g
Dividing by the smallest amount:
N: 0.64/0.32= 2                     Therefore the empirical formula is N2O.
O: 0.32/0.32= 1
Then using the molecular formula:
Molar mass of N2O= 44.0 g/mol
88.0 g/mol= 2                       MF= 2 x N20= N402
44.0 g/mol
Note: When writing down the molecular formula, if it doesn't form an ionic compound, write it alphabetically. 

Time for some random chemistry jokes? Kekekeeke.
Q: What is the chemical formula for the molecules in candy?
A: Carbon-Holmium-Cobalt-Lanthanum-Tellurium or CHoCoLaTe 
Ooooh..trickytricky. ;P
Q: What does a teary-eyed, joyful Santa say about chemistry?
A: HOH, HOH, HOH!
Q: What did the bartender say when oxygen, hydrogen, sulfur, sodium, and phosphorous walked into his bar?
A: OH SNaP!