Molecular Weight

A molecular weight is simply the weight of one mole (6.022×10^23 molecules) of a compound. Like the gram atomic weight, in the case of molecules, we describe the weight of one mole (6.022 x 10^23 molecules) as the gram molecular weight. To calculate the gram molecular weight of a molecule, we add the gram atomic weight of each atom comprising the molecule. For example, to calculate the gram molecular weight of water (H2O), we would add the gram atomic weight of oxygen plus two times the gram atomic weight of hydrogen.

An example of H2O water molecule: The molecular weight is equal to the sum of atomic weights of Hydrogen and Oxygen atoms. Since there are two Hydrogen atoms and one Oxygen. Total is equal to 2(1.0 gram of Hydrogen) + 1(16.0 grams of Oxygen) = 2.0 + 16.0 = 18.0 grams molecular weight. It is also the weight of one mole of water.

Water Compound formed from Hydrogen Atoms and Oxygen Atoms

gram molecular weight of water molecule

Reverse Osmosis membranes will reject ions because of their charge. The membranes will also reject non-charged compounds (like organics) based upon molecular size. Generally, any organic less than 100 molecular weight will not be well rejected. Rejection of organics also depends upon the geometry of the molecule.
Molecular weight is important in organic chemistry since most organic molecules are quite large. Some polymers can have molecular weights in the millions.