Empirical Formulas

      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. Please  take notes as we work through a sample problem.

 Sample Problem:  Molecular Formula Calculation

 Given: 38.7% C,  9.70% H, 51.6% O  and a molecular formula mass of 62.0   find the true 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 Problem #1

Given the following data, find the correct molecular formula:

        24.3% C, 4.1% H, 71.6% Cl   molecular formula mass = 99.0

 To see the correct answer and the method to solve the problem, please continue.

The correct formula would be:   C₂H₄Cl₂                         

The empirical formula would be CH₂Cl   with a formula mass of 49.5                         

Dividing 99.0 by 49.5 gives a factor of 2.  Each subscript is doubled.

Again, check the formula mass of your final answer to confirm your answer.

 

Practice Problem #3

Given the following data, find the correct molecular formula:

      54.6% 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)

Thanks again for working through this program.

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