Molar Mass

We all want to know that in a particular substance how many molecules are present. Molecules and atoms are very tiny, both in size and mass.

What Is Molar Mass?

Relative molar mass is defined as the smallest mass unit of a compound with one twelfth of the mass of one carbon – 12 atom.

In a substance, the amount of entities present for e.g. atoms, molecules, ions, is defined as a mole. A mole of any substance is \(6.022×10^{23}\) molecules. Just as we take a standard value to calculate different things e.g. 1 dozen =12 items similarly we use the mole to calculate the size of the smallest entities quantitatively.

Mole Concept

The amount of atoms present in 12g (0.012 kg) of the \({12}^C\) isotope is the amount of particles present in 1 mole of the substance. One of the most important facts that should be kept in mind is that the mole of a substance always contains the same number of entities whatever the substance may be.

To know the number precisely the mass of carbon-12 atom was calculated by using a mass spectrometer and it was found to be \(1.992648×10^{-3}g\).

As we know that one mole of carbon weighs 12g, the total no. of atoms in it is equal to \(6.0221367×10^{23}\). The no. of entities in 1 mole plays a vital role in calculations in chemistry. That is why it is given a name called Avogadro’s number(NA). From the above discussion, we can say that the number of atoms present in 1 mole of hydrogen is equal to \(6.022×10^{23}\).

Mole Definition

In chemistry the mole is a fundamental (SI) unit used to measure the amount of substance. This quantity is sometimes referred to as the chemical amount.

A substance is something that has mass and occupies space. The molar mass/molecular weight is actually the sum of the total mass in grams of the atoms present to make up a molecule per mole. The unit of molar mass is grams/mole.

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What is the Percentage of Composition?

We can find the percentage composition of a substance by dividing the mass of that substance to the total mass of the substance. Suppose we have to find out the percentage composition of hydrogen in butane(\(C_4H_{10}\)) then it will be:

The total mass of one mole of butane =58.123

Mass of hydrogen in one mole of butane = 10.0794

Therefore, mass percent of hydrogen in butane=\( \frac{10.079}{58.123}×100\)=\(17.3 %\)

These concepts play a very important role in studying the behavior of matter under different conditions.

Solved Example


The molar mass of acetic acid as determined with a mass spectrometer is about 60g/mol. Using that information along with the simplest formula, determine the molecular formula of acetic acid.


First calculate the molar mass corresponding to the simplest formula CH2O then find the multiple by dividing the actual molar mass, 60g/mol by the molar mass of CH2O.

MM CH2O = 12.01 g/mol + 2(1.008 g/mol) + 16.00 g/mol
MM CH2O = 30.03 g/mol

The ratio of the actual molar mass, 60 g/mol to that of CH2O is
(60 g/mol)/ (30.03g/mol) = 2

Molecular formula = C2H4O2


1. Why do we need Mole concept?
Ans: It allows the chemist to weigh quantities of two substances, say iron and sulfur, in order to obtain equal numbers of iron and sulfur atoms. A mole of a substance is known as a material mass containing the same number of basic units as atoms in exactly 12,000 g of 12C.

2. Why is Avogadro’s number called a mole?
Ans: The density of one mole in grams is the weight in atomic mass units of that element. The French physicist Jean Perrin called the number of units in the sum of one mole Avogadro a few years later. For example, one mole of water molecules contains 6.022140758 x 1023 molecules.

3. What is the use of the mole concept?
Ans: The mole is the chemical quantity unit. This connects the atom with the macroscopic quantities of material with which we work in the laboratory. It allows the chemist to weigh quantities of two substances, say iron and sulfur, in order to obtain equal numbers of iron and sulfur atoms.

4. What does Avogadro’s law state?
Ans: The law of Avogadro, also referred to as the rule of Avogadro or the theory of Avogadro, is an experimental gas law that relates the volume of a gas to the amount of gas present. The law of Avogadro states that “equal quantities of all gases have the same number of molecules at the same temperature and pressure.”

5. How many moles are in a mole?
Ans: The mole, abbreviated mol, is an SI unit that measures a specific substance’s number of particles. One mole is equal to 6.02214179 or other elementary units like molecules.

Know more about the mole concept and the related topics, for any further help contact the mentors at Byju’s.

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