MOLECULAR FORMULAS

To find the molecular formula, one more piece of information must be given - the molar mass (also called molecular mass or formula mass).

EX. 1-  An organic compound is found to contain 92.25% carbon and 7.75% hydrogen.  If the molecular mass is 78, what is the molecular formula?

STEP 1:  Find the empirical formula.
C:    92.25 g C | 1 mole C = 7.69 moles C                        H:    7.75 g H | 1 mole H = 7.75 moles H
                      | 12 g C                                                                       | 1 g H

7.69 moles C   = 1             7.75 moles H = 1 
7.69                                  7.69

So...    empirical formula is CH.

STEP 2:  Find molar mass of the empirical formula.
C:    1 x 12.0 = 12.0
H:    1 x 1.0   = 1.0
            MM =   13.0

STEP 3:  Find "multiple" number.
                                    MM of molecular formula = multiple #                              78 = 6
                                    MM of empirical formula                                                 13

STEP 4:  Write molecular formula.
Multiply "multiple" # by all subscripts in the empirical formula.
So...      molecular formula is C6H6.

PRACTICE - An oxide of nitrogen contains 30.4 % nitrogen and 69.6 % oxygen.  If the molar mass of this compound is 92 g/mole, what is the molecular formula?

 

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