Mole Concept And Its Value

       We have already learnt that entities of substances like atoms, ions, molecules, electrons etc are extremely very small in size and their numbers (atoms,molecules) is extremely very large even in a small substance. 

The word Mole is derived from a Latin word “moles” which means “heap” or “pile” which means heap of atoms or molecules. The word mole was used for the first time in 1967 to provide a simple way to calculate large number of atoms in a small substance.

Now, Mole is used as unit of measurement in the International System of Units (SI) for a substance. As we use pair, dozen, bunch etc to measure the quantity of the substance, mole is also used to measure the specific number of particles.

One mole of the substance is an amount of substance containing Avogadro’s number of particles. Avogadro number or Avogadro constant (N_A) is equal to 602,214,199,000,000,000,000,000. It can be simply taken as 6.022 x 10²³. 

A mole or Avogadro number is the number of particles that are exactly equal to number of atoms present in 12 grams of Carbon-12 isotope. 

Use Of Mole Concept In Calculations:

1.Molarity= Number of moles of solute/Volume of solution (litres)

2.Molality= Number of moles of solute/Weight of solvent (kg)

3.Mole Fraction(X) = Fraction of substance in a mixture expressed in mol

             X_A= n_A/(n_A+n_B); X_B= n_B/(n_A+n_B)

             X_A+X_B=1

        (n= number of moles)

Molar Mass And Molecular Mass:

Molar mass is defined as the mass of 1 mole of substance expressed in grams.

Molecular mass or molecular weight is defined as the sum of the atomic masses of atoms in the molecule. Molecular mass has unified mass units.

Example- H₂O

    Molar mass of water (H₂O)= 18 grams.

       18 grams water has 1 mole molecules of water which is 6.022 x 1023 molecules.

    Molecular mass of water (H₂O)= 18 u.

     18 u of water has only one molecule of water.

Dalton Atomic Theory

    In the early 19th century, scientists found out that the elements are following certain laws while combining with other elements. Scientists were confused why the elements are not combining in the arbitrary proportions but only in fixed proportions.

Many scientists tried to give appropriate answers for this behavior of elements. At last an English school teacher JOHN DALTON proposed the basic theory about this nature of matter. His laws are as follows-

1.If the mass to be conserved, then all the elements must be made of very extreme tiny particles called Atoms.

2.If the law of constant proportion to be followed then the substance or matter should contain same type of particles in it throughout it.


Based on the above postulations, Dalton proposed a new system of chemical philosophy known as “Dalton Atomic Theory”.

Postulates of Dalton Atomic Theory:

1.Matter consists of indivisible particles called ATOMS.

2.Atoms are neither created nor destroyed in a chemical reaction. Chemical reaction makes reorganization or rearrangements of atoms.

3.All the atoms of given element have identical mass and will have identical properties. Atoms of different element have different mass and will have different properties.

4.Compounds are formed when atoms of different elements combined together in simple whole number ratios.

5.When atoms of different elements combine together in different whole number ratios, they form different compounds.