Empirical & Molecular Formulas

EMPIRICAL FORMULAS: 

  The simplest formulas of compounds are called Empirical Formulas. They do not show the actual number of atoms but rather the simplest ratios the compound has.

EXAMPLE:

Where Nitrogen can be written as N2 when alone, it's empirical formula would just be N.

Similarly, where Dinitrogen Tetraoxide is written as N2O4, it's empirical formula would just be NO2.

  To determine the empirical formula, we need to know the ratio of each element. For that we will need to prepare a table to help us:

Atom | Mass | Molar Mass | Moles | Smallest Mole | Ratio 

EXAMPLE:

Let's start with 4.58 g of Silicon and 5.21 g of Oxygen.*

*You can find this example and others on: http://homepage.mac.com/sklemmer/Honors/empformulawkst.htm 

1. Create a table.

 Atom | Mass | Molar Mass | Moles | Smallest Mole | Ratio 

2. Fill in the table with what you know. To figure out the number of moles, find the molar mass of each element.

Atom | Mass | Molar Mass |   Moles   | Smallest Mole | Ratio 

Si       4.58 g   28.1 g/mol    0.163 mol

O        5.21 g   16.0 g/mol    0.326 mol

3. To fill in the column for the ratios, you will need to use your smallest mole. What this means is that you take your smallest value of moles which would be 0.163 mol in this case, and divide it from the number of moles of each atom.

Atom | Mass | Molar Mass |   Moles   | Smallest Mole | Ratio

Si       4.58 g   28.1 g/mol    0.163 mol÷0.163= 1          -> 1

O        5.21 g   16.0 g/mol    0.326 mol÷0.163= 2          -> 2

4. Write out the final formula with the ratios in the place of the subscripts. This will be the empirical formula.

SiO2

MOLECULAR FORMULAS:

  While empirical formulas give the simplest ratios, Molecular Formulas give the actual number of atoms. To know the molecular formula, you would need to know the empirical and molar mass.

EXAMPLE:

Empirical   |   Molecular 

C2H6O                ?

     ?              138 g/mol

1. Calculate the molar mass of the Empirical Formula and place it in the table.

Empirical   |   Molecular 

C2H6O                ?

46 g/mol         138 g/mol

2. Figure out the number of times 46 goes into 138 to find out the multiplier.

138÷46= 3

3. Use the multiplier as a co-efficient for the empirical formula and expand. This will give you the molecular formula.

Empirical        |       Molecular 

C2H6O-> 3(C2H6O) -> C6H18O3

46 g/mol               138 g/mol

*For more information on Empirical and Molecular Formulas check out this link:

http://www.youtube.com/watch?v=gfBcM3uvWfs