• A picture of a mole holding an Erlenmeyer flask.

    Textbook Reading: 

    Holt "Chemistry" Ch. 7 S. 3 p 241-245
    Silberberg "Chemistry" Ch.3 S.2

    A Molecular Formula Is 

    • The actual number of atoms in a molecule.
      • The molecular formula of propane is C3H8.
      • One mole of propane contains 3 moles of carbon atoms and 8 moles of hydrogen atoms.
      • One molecule of propane is made of 3 carbon atoms and 8 hydrogen atoms covalently bonded together.
    • Able to be used to calculate the molar mass of a molecule by adding up the masses of the atoms in the formula.
      • The molar mass of propane is (3 x 12.011) + (8 x 1.0079) = 44.096 g/mol.
      • One mole of propane has a mass of 44.096 g.
    • Not as informative as a full structure.
      • The full structure of propane shows how the atoms bond together
      • Lewis structure of Propane
      • Sometimes one molecular formula can describe multiple compounds.  These are called isomers.
        •  There are 3 isomers of C5H12, known as 'pentanes'.
        • 3 isomers of C5H12

    An Empirical Formula is

    • A formula giving the lowest ratio of atoms in a molecule.
      • The molecular formula of water is H2O.  The empirical formula is also H2O.
      • The molecular formula of hydrogen peroxide is H2O2, but the empirical formula is simply HO.
      • The molecular formula of ethane is C2H6, but the empirical formula is CH3.
    • A formula giving the lowest ratio of moles of atoms in a compound.
      • Water has 2 moles of H for each mole of O.
      • Ethane has 3 moles of H for each mole of C.
    • Not as informative as a molecular formula.
    • Can be readily determined from the masses or percentages of different elements in a compound.
      • This can be found by actual experiment in many cases.
      • Empirical means 'by experiment'.

    Use the Molar Mass to Convert from Empirical Formulas to Molecular Formulas

    A compound has the empirical formula of CHO2 and a molar mass of 90.0 g/mol.  What is the molecular formula?

     (1 x 12.011) + (1 x 1.008) + (2 x 15.9994) = 45.018 g/mol
     
        90.0 g/mol = 2
      45.018 g/mol
     
     Molecular formula is 2 x (CHO2) = C2H2O4

    Find the Simplest Integer Ratio of Moles to Determine an Empirical Formula 

    A compound has molecular formula of C12H10.  What is the empirical formula?

     Both 12 and 10 have a common factor of 5.

     The empirical formula is C6H5

    A sample of an unknown compound contains 0.21 mol Zn, 0.14 mol P, 0.56 mol O.  What is the empirical formula?

    Of the 3 amounts, the smallest is 0.14 mol.  Divide all the amounts by 0.14 mol.

     0.21 mol Zn / 0.14 mol = 1.5 Zn
     0.14 mol P  / 0.14 mol = 1 P
     0.56 mol O  / 0.14 mol = 4 O

     We can't use decimals in formulas!  
     We notice that 0.5 = 1/2, so we multiply everything by 2
     Zn(1.5)P(1)O(4) x 2 = Zn3P2O8
     The empirical formula is Zn(1.5)P(1)O(4) x 2 = Zn3P2O8

    A sample of an unknown compound contains 2.82 g Na, 4.35 g Cl, 7.83 g O.  What is the empirical formula?

     Formulas relate moles, so convert grams to moles.
    HINT:  Keep extra decimal places, round at the end!
    This avoids calculation errors.
    2.82 g Na x 1   mol Na  =  0.12266 mol Na
                 22.99 g Na

     4.35 g Cl x 1   mol Cl  =  0.12270 mol Cl
                 35.45 g Cl

     7.83 g O x 1   mol O  =  0.48938 mol O
                16.00 g O

    Smallest is 0.12266 mol so divide:
    0.12266 mol Na / 0.12266 mol = 1.00 Na
    0.12270 mol Cl / 0.12266 mol = 1.00 Cl
    0.48938 mol O  / 0.12266 mol = 3.99 O

    We rounded to 3 sigfigs because that's what we originally had.
    Error in the last place is not important (rounding!):

    Empirical formula is NaClO4
     

    An unknown hydrocarbon contains 6.291% hydrogen by mass and has molar mass of about 128 g/mol.  What is the molecular formula?

    HINT:  We can pretend our sample is 100 grams just to make it simple.
    The sample size doesn't affect the formula:  a drop of water and a 
    swimming pool are both H2O!
    6.291 g H x 1    mol H  =  6.2417 mol H
                1.0079 g H

    We know percentages must add up to 100% so 93.709% carbon:
     93.709 g Cl x 1    mol C  =  7.8019 mol C
                   12.011 g C


    6.2417 mol H / 6.2417 mol = 1.00 H
    7.8019 mol C / 6.2417 mol = 1.25 C

    We notice that 0.25 is 1/4, so multiply everything by 4.

    Empirical formula is C5H4

    Molar mass of C5H4 is (5 x 12.011) + (4 x 1.0079) = 64.08 g/mol

    128 / 64 = 2

    Molecular formula is C10H8

    Check for understanding!  Take the test here! As before, this is worth extra credit in your TEST category.