Molecular Formula 2

Molecular Formula: The actual formula for a molecule.

Problem:

A compound is 75.46% carbon, 4.43% hydrogen, and 20.10% oxygen by mass. It has a molecular weight of 318.31 g/mol. What is the molecular formula for this compound?

Strategy:

1. Find the empirical formula.
   
   
   • Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages).
     
     (.7546) (100 g) = 75.46 g C

     (.0443) (100 g) = 4.43 g H

     (.2010) (100 g) = 20.10 g O
     
     

   • Convert the mass of each element to moles.
     
     (75.46 g C) (1 mol/ 12.00 g C) = 6.289 mol C

     (4.43 g H) (1 mol/ 1.008 g H) = 4.39 mol H

     (20.10 g O) (1 mol/ 16.00 g O) = 1.256 mol O
     
     

   • Find the ratio of the moles of each element.
     
     (1.256 mol O)/ (1.256) = 1 mol O

     (6.289 mol C)/ (1.256) = 5.007 mol C

     (4.39 mol H)/ (1.256) = 3.50 mol H
     
     

   • Use the mole ratio to write the empirical fomula.
     
     Multiplying the mole ratios by two to get whole number, the empirical formula becomes:

     C₁₀H₇O₂
     
     

2. Find the mass of the empirical unit.
   
   10(12.00) + 7(1.008) + 2(16.00) = 159.06 g/mol
   
   
3. Figure out how many empirical units are in a molecular unit.
   
   (318.31 g/mol) / (159.06 g/mol) = 2.001 empirical units per molecular unit
   
   
4. Write the molecular formula.
   
   Since there are two empirical units in a molecular unit, the molecular formula is:

   C₂₀H₁₄O₄