Atomic weight is the averaged mass of the isotopes of elements as they appear in nature.  These values are listed in any chemistry textbook; they also appear on the periodic table in the box for each element.

one moles of Si atoms = 28.08 g

Molecular weight (also called formula weight) is the sum of the masses of all the atoms in a compound.  It is the mass, in grams, of one mole of a compound. 

Example:  Na2CO3, sodium carbonate.  One mole contains:

    2 moles of Na atoms = 2 x 23 (at. wt.) = 46 grams

    1 mole        C  atoms = 1 x 12 (at. wt.) =  12 grams

    3 moles      O  atoms = 3 x 16 (at. wt.) =  48 grams

                molecular weight of Na2CO3  = 106 grams

Calculating Molar Masses

Formula of a Hydrate

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