What is molecular weight? Definition

However, in chemistry , the term molecular weight refers to the molecular mass , which is the sum of the masses of all the atoms of an element. Molecular mass is a measure of magnitude that indicates how many times the mass of a molecule is greater than the atomic mass unit. It is expressed in atomic mass units (amu).

How Molecular Mass is Calculated

To know how to calculate the molecular mass, it is necessary to take into account the following concepts:

• Isotopes : are atoms of the same element that have a different number of neutrons, and therefore vary in mass.
• Atomic weight : is the average weight of all the isotopes of an element. It appears on the periodic table of elements and is expressed in atomic mass units (amu).

The calculation of the molecular mass is made according to the molecular formula of the compound. The number of each type of atom is multiplied by its molecular mass and then added to the masses of the other atoms.

For example, the molecular formula of hexane is C ₆ H ₁₄  .  The subscripts indicate the amount of each type of atom present in the molecule; therefore, in this molecule we have 6 carbon atoms and 14 hydrogen atoms . The respective molecular masses of carbon and hydrogen can be found on the periodic table:

• Atomic mass of carbon (C): 12.01
• Atomic mass of hydrogen (H): 1.01

So, the mass of this molecule is calculated like this:

Molecular mass = (number of C atoms) (atomic mass of C) + (number of H atoms) (atomic mass of H)

Therefore:

The molecular mass of hexane = (6 x 12.01) + (14 x 1.01)

= 72.06 + 14.14

= 86.20 uma

Atomic mass and isotopes

Keep in mind that if you are working with specific isotopes of an atom, you must use the average atomic mass of the isotope indicated in the periodic table. The difference between the atomic mass of an element and the atomic mass of a specific isotope is usually small, but it can be important in certain calculations.

Bibliography

• Miglietta, A. Atoms and molecules. Brief history of chemistry . (2019). Spain. susaeta.
• Jackson, T. The Periodic Table, A Visual Guide to the Elements. (2020). Spain. Anaya Multimedia.