When
we mix two substances, we get one or more new substance(s).
For example when we mix hydrogen and oxygen and ignite the
mixture, we get a new substance water. This can be represented
in the form of an equation,
2H2 (g) + O2(g) ® 2H2O
(l)
In above equation, 2 molecules (4 atoms) of hydrogen react
with 1 molecule (2 atoms) of oxygen and give two molecules
of water. Similarly, we always like to know how many atoms/molecules
of a particular substance would react with atoms/molecules
of another substance in a chemical reaction. No matter how
small they are. The solution to this problem is to have
a convenient unit of matter that contains a known number
of particles (atoms /molecules). The chemical counting unit
that has come into use is the mole.
The word mole was apparently introduced in about 1896 by
Wilhelm Ostwald who derived the term from the Latin word
‘moles’ meaning a ‘heap’ or ‘pile’.
The mole whose symbol is ‘mole’ is the SI base
unit for measuring amount of substance. It is defined
as follows:
‘A mole is the amount of pure substance that contains
as many particles (atoms, molecules, or other fundamental
units) as there are atoms in exactly 0.012 kg of C-12 isotope’.
In simple terms, mole is the number of atoms in exactly
0.012 kg (12 grams) of C-12. Although mole is defined in
terms of carbon atoms but the unit is applicable to any
substance just as 1 dozen means 12 or one gross means 144
of any thing. Mole is scientist’s counting unit like
dozen or gross. By using mole, scientists (particularly
chemists) count atoms and molecules in a given substance.
Now it is experimentally found that the number of atoms
contained in exactly 12 grams of C-12 is 602,200 000 000
000 000 000 000 or 6.022×1023. This number
(6.022×1023) is called Avogadro constant
in honour of Amedeo Avogadro an Italian lawyer and
physicist and is denoted by symbol, NA. We have seen that
Atomic mass of C = 12 u
Atomic mass of He = 4 u
We can see that one atom of carbon is three times as heavy
as one atom of helium. On the same logic 100 atoms of carbon
are three times as heavy as 100 atoms of helium. Similarly
6.02×1023 atoms of carbon are three times
as heavy as 6.02 × 1023 atoms of helium.
