Chapter 7


Molecular mass is the sum of the atomic masses of the atoms of each element in a molecule.

Mole - the measure of quantity of a substance.

1 mole of any substance has the amu of that substance

1 mole of any substance has 6.0225 X 1023 atoms, molecules, ions of that substance

amu of a substance = 1 mole of that substance = 6.0225 X 1023

Change moles to grams:

Change moles to grams:

0.62 mol NaOH

40.01 g NaOH

 

1 mol NaOH

Change grams to moles:

2.5 g NaOH

1 mol NaOH

 

40.01 g NaOH

Change moles to atoms/molecules:

0.62 mol NaOH

6.02 x 1023 atoms/mlc NaOH

 

1 mol NaOH

Change atoms/molecules to moles:

2.35 X 1022 atoms/mlc NaOH

1 mol NaOH

 

6.02 X 1023 atoms/mlc NaOH

Change from atoms/molecules to grams:

2.35 X 1022 atoms/mlc NaOH

1 mol NaOH

40.01 g NaOH

 

6.02 X 1023 atoms/mlc NaOH

1 mol NaOH

Change from grams to atoms/molecules:

2.35 g NaOH

1 mol NaOH

6.02 x 1023 atoms/mlc NaOH

 

40.01 g NaOH

1 mol NaOH



Percent Composition

Calculate the percent composition of Ba(OH)2.

  1. Calculate the amu of the compound.
    
                    Ba  =  1   x  137.33  =  137.33
                      O  =  2   x   16.0   =   48.0
                      H  =  2   x    1.01  =    2.02
                                                 _________
                                                     187.35
  2. Divide the total amu of each element by the total amu for the compound.



                137.33  187.35  =  0.733 x 100 = 73.3% Ba

                48.0    187.35  =  0.256  x  100  =  25.6% O

                2.02    187.35  =  0.011  x  100  =  1.1% H

Empirical and Molecular Formulas


Change % to grams               Change grams to moles               Divide by the lowest               Multiply to get whole #'s

    63.1% Mn       =  63.1 g Mn  x 1 mol Mn     =     1.15  1.15  =  1          (not necessary in this example)
                                                    54.94 g Mn    
                              
    36.9% S         =  36.9 g S   x   1 mol S      =      1.15  1.15  =  1          (not necessary in this example)
                                                  32.07 g S

Empirical Formula  =  MnS
Use the same procedures for the molecular formula and add the last steps.

Change % to grams               Change grams to moles               Divide by the lowest               Multiply to get whole #'s

    92.25% C       =  92.25 g C  x 1 mol C     =     7.68  7.67  =  1          (not necessary in this example)
                                                 12.01 g C    
                              
    7.75% H         =  7.75 g H   x   1 mol H     =    7.67  7.67  =  1          (not necessary in this example)
                                                  1.01 g H

Empirical Formula  =  CH

molecular mass  =  78

[EFamu]x   =   molecular mass

[13.02]x   =   78    (solve for x)

13.02x  =  78
        x  =  6

plug in x and multiply through the empirical formula subscripts

[CH]6      =    C6H6  =  molecular formula



Practice Problems:

A chemist has a jar containing 388.2 g of iron filings. How many moles of iron does the jar contain?

A student needs 0.366 mol of zinc for a reaction. What mass of zinc in grams should the student obtain?

How many moles of lithium are there in 1.204 x 1024 lithium atoms?

How many boron atoms are there in 2.00 g of boron? atoms B)

How many moles of carbon dioxide are in 66.0 g of dry ice, which is solid CO2?

Determine the number of molecules in 0.0500 mol of hexane, C6H14.

Calculate the mass of 8.25 x 1022 mlc of bromine pentafluoride.

Calcule the number of atoms in 22.9 g of sodium sulfide.

Calculate the number of grams in 3.0 x 1021 mlc of CH2CHCN.