Converting Grams to Mole 
This is probably the single most used equation in chemistry.  It is the one that allows the conversion of moles into grams and grams back into moles.  Moles are a "theoretical value" which looks good on paper.  Grams is the "practical value" that you take out of a stock bottle and place on the balance.  Lab balances read in grams, not moles, and so in practical work all mole values have to first be converted.

Moles to grams formula:     grams(g)  = moles(n)  X  molecular mass

Example:  Sodium bicarbonate, NaHCO3, is one ingredient of baking powder.  How many grams of sodium bicarbonate are in 0.673 moles?
 
Step 1.  Find the molecular mass:
          NaHCO3 = 1 Na = 1 X 22.99 g/mol =  22.99 g/mol
                             1 H   = 1 X   1.01 g/mol =    1.01 g/mol
                               1 C = 1 X 12.01 g/mol =  12.01 g/mol
                               3 O = 3 X 16.00 g/mol =  48.00 g/mol
                                                                       84.01 g/mol
 
Step 2. Use the equation:   g = n  X  molecular mass
                                            = 0.673 mol X 84.01   g
                                                                              mol
                                            = 56.54 g

Step 3: Provide a written answer:  There are 56.54 grams of sodium bicarbonate in 0.673 moles.
 
Grams to moles formula:

 moles =         grams          OR  n =            g
                molecular mass                 molecular mass


Example:  Potassium permanganate, KMnO4, at one time was used for an anti-fungal agent. You
                could always tell someone who had just been treated because their feet were purple. The
                pharmacy gives you 250 grams of the stuff in a bottle.  How many moles of it do you
                have?
 
Step 1:  Find the molecular mass of KMnO4:
              1  K  = 1 X 39.10 g/mol  =  39.10 g/mol
              1 Mn = 1 X 54.94 g/mol  =  54.94 g/mol
              4 O   =  4 X 16.00 g/mol  = 64.00 g/mol
                                                       158.04 g/mol
 
Step 2:  Use the equation:  n =           g             =         250 g          =  1.58 mol
                                               molecular mass      158.04 g/mol
 
Step 3:  Write a written answer.
A sample of 250 grams is equivalent to 1.58 moles of potassium permanganate.

Finding the molecular mass
In the above two examples you have had a molecular formula to work with.  As long as you have a molecular formula, or name, from which you can make a formula up, you will always have a molecular mass.   In order to find the molecular mass, without a name to go on, you need two pieces of information. The number of grams and the number of moles that it represents.
 
        molecular mass =    grams     =        g
                                     moles               n
 
Example:   A fellow student comes to you with a sample of an unknown chemical. They tell you that it has a mass of 34.91 grams and that it is exactly 0.20 moles.   What is the molecular mass of the substance?

Step 1:  Use the equation:     molecular mass =   34.91  g   =  174.55     g
                                                                                0.20 mol                    mol

Step 2: Write a sentence.  The unknown substance has a molecular mass of 174.55 g/mol.
          Go to the Grams, Moles and Molecular Mass Worksheet
          Marvin Da Mole Worksheet
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