Empirical & Molecular Formulas!!!

Empirical Formula :  help state the lowest ratio of atoms(or moles) inside a formula

*ALL IONIC COMPOUNDS ARE EMPIRICAL FORMULAS*

For example: C₁₂H_{10 Acnaphthene's empirical formula would be C6H5}

To convert molecular formulas to empirical formulas, use this :

1. Convert grams to moles

2. Divide each mole by the lowest molar amount

3. Scale the ratios to whole numbers

An example would be like this:

Calculate the empirical formula of a compound that contains 65% nitrogen and 35% fluorine.

*assume that it adds up to 100 grams*

Step 1: N:  65g x 1 mol    = 4.643

                                     14 g

            _{F: 35g x 1 mol     = 1.842}

                           _19g

_{Step 2:              4.643   = 2.52}

                         1.842

                          1.842   = 1

                          1.842

Step 3: Since 2.52 isn't a whole number, we need to scale it. Multiply each number by a whole number until 2.52 is a whole number .

In this case, 2.52 x 2 = 5.04<----since it's so close you can round down to 5.

Don't forget to multiply 2 to the other number as well . 2 x 1 =2

N5F2 is the empirical formula.

Tip: if the numbers are already whole, just use them as the empirical formula.

Molecular Formula (MF): is the multiple of an empirical formula that shows the actual # of atoms combined to form a molecule.

Formula :   n=      molar mass of compound 

                         molar mass of empirical formula 

Which means that MF= empirical formula x n

Example: A compound contains a empirical formula of C2H3 with a molar mass of 54 g /mol. Find the molecular formular.

Molar mass of C2H3= 27 g/mol

n=   54g/mol  =2

       27g/mol

MF= 2(C2H3) = C4H6


By : JN