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.