Molar Mass to Molecular Mass

Finding the molecular mass of a molecule is as simple as adding the molar mass of all the involved elements.  Molecular mass is actually a misnomer and some tend to use it interchangeably with molecular weight.  I'm not a chemist, so they both mean the same thing to me and I've seen both terms used.

First of all, a molecule is the combination of more than one atom (of the same or different elements) to form a substance. H (hydrogen) is an element.  H2O, O2 and HCl are molecules.

The molecular mass is simply the weight of one molecule. 

To find the molecular mass, simply add up the weights of every individually present element. 

For example, a molecule of H2O contains 1 atom of Hydrogen and 2 atoms of  Oxygen. 

 

 If you look at your periodic table you will see the atomic mass for each element under the chemical symbol.  

(Note:  Your periodic chart likely has atomic masses that extend several places past the decimal. The numbers here are rounded up.  Use the weights that you see in your book, the concept is the same.) 

Alright.  The molecular mass of one molecule of H2O is 18 g/mol.  There are 2 Hydrogen atoms (1 gram x2=2) and 1 Oxygen atom (16 grams).  2+16=18.  

Let's do the same with O2.  There are 2 atoms of Oxygen (16 g x 2) in this molecule, so its molecular weight is 32 g/mol. 

One more?

HCl has one atom of Hydrogen (1g) and one atom of  Chlorine.(35 g), so the molecular weight of HCl is 36g/mol. (1 g +35 g)

 

It's really not as intimidating as it may seem.  Just add up every element in the molecule. Your biggest problem may be overlooking the actual number of atoms, so be careful.