Moles and Molar Mass

Difference Between Molar Mass and Atomic Mass

To summarize, molar mass is an umbrella term referring to either the average masses of an element per mole (atomic weight) or compound per mole (molecular weight). The atomic mass does not fall under the molar mass umbrella since it is not an average, but the mass of a single element/isotope.

Molecular Mass and Molar Mass

Using atomic masses, we can calculate the molecular mass.

The molecular mass is the sum of the atomic masses of the elements present in the molecule.

Molecular mass is different from molecular weight.

Molecular weight is the sum of the atomic weights of the elements present in the molecule.

As I mentioned before, atomic mass and atomic weight are often used interchangeably, so molar mass, molecular mass, AND molecular weight are also often used interchangeably. Thus, the context and questions must be clear when discussing these terms! Always be careful when reading textbooks and problems to see if when "molecular mass" is mentioned they actually mean "molecular mass" or if they mean "molecular weight" instead.

As a guide to get you used to this, if I am using a term with its original definition, it will be in green, if I am using it with its colloquial definition it will be in red.

The molecular weight of water can be calculated by adding together the atomic masses of the elements in it. Water is made up of 2 hydrogens and 1 oxygen, and the atomic mass of hydrogen is 1.01 g/mol and the atomic mass of oxygen is 16.00 g/mol. The molecular mass would then be: $$(2*1.01\frac{g}{mol})+16.00\frac{g}{mol}=18.02\frac{g}{mol}$$

It is important to remember that atomic weights used in the calculations above are averages for their respective element. Our calculated value, therefore, represents an average molecular mass for a given molecule of water. There are 2 stable isotopes of hydrogen and 3 for oxygen, which makes the exact molecular mass for 1 molecule of water vary greatly!

Difference Between Molar Mass and Molecular Mass

The amount of swapping between terms can make it hard to tell the difference between them. The main thing to remember is that the molecular mass is the mass of that specific instance of the molecule, while the molar mass is the average of all instances of that molecule based on their abundance. Molecular weight is the term that is synonymous with molar mass.

Chart showing the relationship between terms. StudySmarter Original.

Molar mass and boiling point

Several factors affect a substance's boiling point, one of them being the molar mass. In general, the larger the molar mass, the larger the boiling point.

Boiling point is also dependent on atomic mass. Since isotopes have different masses, they will also have different boiling points. Like with the general trend, the greater the atomic mass, the greater the boiling point.

Molecules follow the same general trend, however, just because a molecule is heavier, doesn't necessarily mean it will have a higher boiling point. For example, the boiling point of ethanol (CH₃CH₂OH, molar mass=46.07 g/mol) is 78.4 °C, however, water (molar mass=18.02 g/mol) has a boiling point of 100 °C. This is due to intermolecular forces, which are the forces that exist between molecules. The stronger these forces are, the harder it is for molecules to "pull away" from each other and enter the gas phase.

It is more accurate to compare the molar masses of like molecules, such as in the graph below:

Relationship between molar mass and boiling point for different molecule types. Wikimedia Commons.

Here we see that when molecules are in the same "class", an increase in molar mass means an increase in boiling point.