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Last modified: December 9, 2021 ✓

Avogadro's Number
-----------------

One Mol is the number of atoms in 12 g of Carbon-12.  
This number is called Avogadro's Number, NA and is 
equal to:

           6.022 x 1023

Atomic Mass Unit(amu)
---------------------

One amu (u) is defined as 1/12 of the mass of an 
unbound neutral atom of Carbon-12 in its nuclear 
and electronic ground state.

u = 1.660538921(73)×10-24 grams

Note: u is simply the inverse of Avogadro's number 
expressed in grams.

  = 1/(6.022 x 1023) grams

Molar Mass
----------

The molar mass of any molecular is the sum of the 
atomic masses of the atoms in the molecules.

Atomic Weight
-------------

The atomic weight of an element is the weighted 
average of the atomic masses of the different 
isotopes of an element.

Formulas
--------

# of Mols = Mass (g)/Molar Mass (g/M) 

# of molecules/atoms = # of Mols x 6.022 x 1023

Examples
--------

1.  What is the molar mass of Carbon Tetrachloride, 
    CF4?

    MM of C = 12
    MM of F = 19

    12 + 4(19) = 88 g/mol

2.  Which is the formula for the compound whose 
    percent composition 47.51% O, 38.62% K and 
    13.87% N?

    # of Mols = Mass/Molar Mass

    MM of O  = 16
    MM of K  = 39
    MM of N  = 14

    O: 47.51/16 = 2.97 Mols
    K: 38.62/39 = 0.93 Mols
    N: 13.87/14 = 0.99 Mols

    Compute the ratios:

    N:  0.99/0.99 = 1 
    K:  0.93/0.99 ~ 1
    O:  2.97/0.99 ~ 3

    Therefore,

    K2NO2

3.  What is the formula for a compound that 
    contains 64.75g Nitrogen and 185.25g Oxygen?

    # of Mols = Mass/Molar Mass
    64.75/14 = 4.6 Mols of Nitrogen
    185.25/16 = 11.575 Mols of Oxygen
    The ratio 11.575/4.6 ~ 2.5 (= 5/2)

    Therefore, N2O5 is the correct formula.

4.  What is the mass of 8 moles of Sodium Chloride?

    Mass = # of Mols x Molar Mass
         = 8 x 58.44
         = 467.52g

5.  How many moles of atoms are in one mole of 
    AlPO4?

    Mols of P  = 1
    Mol2 of O  = 4
    Mols of Al = 1
                ---
                 6
    
5.  How many molecules are in 3.6 grams of NaCl?

    # of molecules = Mass/Molar Mass
                   = 3.6/58.44
                   = 0.062 Mols

    # of molecules = Mols x 6.022 x 1023
                   = 3.7 x 1022 molecules

6.  How many grams are in 1.946 moles of NaCl?

    Mass = Mols x Molar Mass
         = 1.946 x 58.44
         = 113.8g

7.  What is the percent composition of Phosphorus 
    in Zn3(PO4)2?

    MM of P  = 30.97
    MM of O  = 16
    MM of Zn = 65.38

    % = 2(30.97)/[3(65.38) + 2(30.97 + 4(16))] x 100
      = [61.94/(197.44 + 189.94) = 61.94/387.38] x 100
      = 16%


8.  Calculate the number of molecules in 4.0 Mols of
    H2O.

    # of molecules = # of Mols x 6.022 x 1023
                   = 4 x 6.022 x 1023
                   = 24.088 x 1023 molecules


9.  What is the mass in grams of 1.02 x 1024 atoms 
    of Manganese (Mn)?

    # of Mols = # of atoms/6.022 x 1023              
              = 1.02 x 1024/6.022 x 1023 
              = 1.69 Mols

    Mass = # of Mols x Molar Mass
         = 1.69 x 54.94
         = 93g

 

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