Molecular mass- Definition and calculations

Published by Daniel Ozori on


Molecular mass is actually a pretty simple topic and not as complicated as it looks in most textbooks, well firstly!…

Molecular mass– Definition

The molecular mass of an element or a compound is the number of times the average mass of one molecule of that substance is heavier than one twelfth the mass of one atom of carbon-12 ( carbon-12 is one of the two allotropes of carbon found in nature and the most abundant).

And most people easily get confused by this because they don’t understand the term “one twelfth the mass of one atom of carbon-12”.

Well when we say one twelfth, we mean taking the mass of one atom of carbon-12 and multiplying it by the fraction 1/12.

We can mathematically represent the definition for better understanding like this

average mass of given substance/ 1/12 × the mass of one atom of carbon-12

It’s just one way of saying that it’s the sum of the weight of all the atoms in a molecule of a substance,it is used to determine the amount of atoms in a substance

The standard method for calculating it was first introduced in 1873 and has been used by many scientists ever since.

How to calculate the molecular mass of a compound

We’ll go through some examples to show how successfully calculate the molecular mass of a compound.

When solving these questions, do not get scared by the large numbers or the complexity of the question….just follow the right procedures and you’ll be fine

  • Calculate the relative molecular mass of limestone,CaCO3. (Ca = 40, C = 12, O = 16)


According to the molecular formula “CaCO3” contains 1 Calcium , 1 Carbon and 3 Oxygen atoms.

So therefore, the relative molecular mass of CaCO3 can simply be calculated by multiplying the atomic mass of each atom by their amount in the substance and adding their values.

i.e (1 × 40 for Calcium) + (1 × 12 for Carbon) + (3 × 16 for Oxygen)
= 40 + 12 + 48

= 100 g / mol

Making the molecular mass of CaCO3 (limestone) is 100 g / mol

Lets try a slightly more difficult one,shall we?

  • What is the the molecular mass of C10H22?


C10H22 commonly known as decane (an organic compound) is shown from it’s molecular formula to contain 10 carbon atoms and 22 Hydrogen atoms.

( 10 × 12 for carbon) + ( 22 × 1 for hydrogen)

=120 + 22

= 142 g / mol

Some compounds and their molecular masses

  • trioxonitrate ( v ) acid ( HNO3 )———-63g
  • Limestone ( CaCO3 )———————-100g
  • Methanoic acid ( CH3COOH )————60g

Fun fact: “Glass is actually a liquid,it just flows very slowly”


Anonymous · October 28, 2019 at 9:17 pm


Tips on studying chemistry | Easy Chemistry · December 7, 2019 at 11:00 am

[…] book and study what was taught in class, also practice the essential equations in chemistry like Molecular mass, Mass percentage, and Mole  and this should be done […]

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