The molecular weight of a molecule can be obtained by adding the atomic weights of all the atoms in the molecule. The atomic weight of an element is listed on the periodic table. 

For example, here’s how to calculate the molecular weight of carbon dioxide: 

• 1 molecule of Carbon Dioxide (CO2) is made up of 1 Carbon atom and 2 Oxygen atoms. 
• The atomic weight of carbon is 12.011 
• The atomic weight of oxygen is 15.999 

Hence, molecular weight of carbon dioxide is 12.001 + (15.999 x 2) = 44.009 Da.