6.5- The Empirical formula 

 

what is that?

-the simplest formula for a compound.

-a molecular formula is the same as or multiple of the empirical formula, and is based on the actual number of atoms of each type in the compounds. 

 

example:

 

 

 

how to calculate:

You can use information about reacting masses to calculate the formula of a compound. Here is an example:

 

Question 1:

Suppose 3.2g of sulfur reacts with oxygen to produce 6.4g of sulfur oxide. What is the formula of the oxide?

Use the fact that the Ar of sulfur is 32 and the Ar of oxygen is 16.

 

Here is the calculation again in table form to help you remember the steps:

    

     Step     action               S     O                                     

        

         1       find masses    3.2  3.2

         2     look up given    32   16

                   Ar  values

         3     divide masses  0.1 0.2

                  by Ar

   4      find the ratio    1       2

 

 Result: the formula for the oxide

            = SO2 

 

Note: if final answer is not a whole number, multiply it until it becomes a whole number

            if question gives a percentage instead of mass, assume the overall substance 100g (100%)

 

Example:

 

A compound consists of 72.2% magnesium and 27.8% nitrogen by mass. What is the empirical formula?

 

Step 1. Percentage to mass

     Magnesium:72.2g    Nitrogen: 27.8g

 

Step 2. Mass to moles

     for Mg: 72.2 g Mg x (1 mol Mg/24.3 g Mg) = 2.97 mol 

     for N:   27.8 g N x (1 mol N/14.0 g N) = 1.99 mol 

 

Step 3. Find the ratio (you can do this by dividing the smallest number with both values)

     for Mg: 2.97 mol / l.99 mol = 1.49

     for N:  1.99 mol / l.99 mol = 1.00

 

Step 4. Multiply until a both are whole numbers

     for Mg: 2 x 1.49 = 2.98 (i.e., 3)

     for N: 2 x 1.00 =   2.00

 

 

 

 

 

 

        

           PREVIOUS                                                           NEXT