Empirical and Molecular Formula

Empirical Formula gives the lowest term ratio of atom in the formula


Finding the empirical formula is somewhat the reverse of finding percentage composition. ( see below)
First you will be given the percentages of each element in a particular compound.

Assuming that you will always be given a 100 gram sample we will convert these percentages to grams. The next step is to find the ratio of moles of each element and then calculate the simplest ratio of subscripts that maintain that ratio.

It is much easier done than said!

Lets start with this data: 36.5% Na, 25.4% S, and 38.1% O
First, convert each percentage to grams : 36.5 g Na, 25.4 g  S, 38.1 g O.
Next, divide each by the grams/mole of that element:

Na: 36.5 g / 23.0 g/mol = 1.58 mol  Na
S: 25.4 g / 32.1 g/mol = 0.791 mol S
O: 38.1 g / 16.0 g/mol = 2.38 mol O

Now we can set up the ration of moles of each element:

Na         S       O
1.58   0.791   2.38

To convert these decimal numbers into whole numbers and maintain the same ration between them, just divide each by the smallest of the subscripts.

1.58/0.791 

0.791/0.791

 2.38/0.791

Our final formula would look like:  Na2SO3   ( sodium sulfite)




MORE EXERCISES!!! :)

Molecular Formula is a multiple of the empirical formula 

Molecular Formula = Molar Mass / Molar Mass of empirical formula

 To find the molecular formula, first find the empirical formula for the data you have been given. Then compare the formula mass for your empirical formula with the formula mass for the molecular formula. You may have to multiply the subscripts in your empirical formula by some factor.

Sample Problem.
Given: 38.7% C, 9.70% H, 51,6% O and a molecular formula mass of 62.0 find the molecular formula:

 First, find the empirical formula ===>  CH3O

 Next find the formula mass: C 1 x 12.0 = 12. 0
                                           H 3 x 1.01 = 3.03
                                           O 1 x 16.0 = 16.0
                                  Formula mass  = 31.0

 Now divide the molecular mass by this formula mass : 62.0 / 31.0 = 2
 Multiply each subscript in your formula by this factor ( 2 here)
Your final molecular formula: C2H6O2

 To check your answer find the formula mass of your final formula. It should be the same as the molecular formula mass.

 Practice:
 Given the following data, find the correct molecular formula:
34.6% C , 9.00% H, 36.4% O and a molecular mass of 176

The correct answer would be:  C8H16O4

The empirical formula would be: C2H4O with a formula mass of 44

The multiplication factor would be 4 ( 176/44.0)

ONE MORE TIME!

VIDEO!!!!!!!!!!!!!! (: