The molecular weight (MW), or relative molecular weight, of a molecule can be found by the addition of all the atomic weights of all the atoms that are present in the molecule these can only be molecules which actually exist, and are joined together with covalent bonds. The atomic weight data can be found on the periodic table. The molecular weight can be important in helping to find concentration, after finding the number of moles.
If you were to calculate the MW of glucose;
The molecule is C6H12O6
The atomic mass of carbon is 12.
The atomic mass of hydrogen is 1.
The atomic mass of oxygen is 16.
So to calculate the molecular weight of glucose you would do. (12x6)+(1x12)+(16x6) = 180