2.3 MOLE:
(A practical chemical unit for handling atoms and molecules) Since atoms and molecules are so small, it is impossible to handle and
count atoms and molecules individually. Therefore, the Chemists devised a special unit to describe very large number of atoms, ions and molecules.

It is called mole and is abbreviated as mol. A mole can be defined as “the molecular mass, atomic mass and formula mass of a substance expressed in grams. Thus, 12g of carbon is equal to 1 mol of carbon atoms. 24g of C is equal to 2 mol of carbon atoms" The mole concept tries to give a practical meaning to the mass of reactants and products in chemical reactions in terms of the number of particles (atoms, molecules or ions) involved. The actual countable number of particles in one mole of a substance is 6.02 x 1023 particles, which is referred as Avogadro number.

The (S.I.) definition of mole is the amount of substance, containing as many elementary particles (units) as there are atoms in exactly 12g of C-12 a.m.u.

It is also defined as the mass of any substance equal to its atomic mass, molecular mass or formula mass in grams.


1 mole of C                   =          12 g

1 mole of Mg                 =          24 g

1 moleofH2O                  =          18 g

1 mole of C02                 =          44 g

1 mole of CaCO3            =          100g

1 mole of Fe203 =          160g

By formula, Number of moles                 =          Give mass of substance

                                                                        Atomic mass or formula mass

As, formula mass, represents the both covalent and electrovalent compounds

 2.3.1 Avogadro's Number (N4):

A mole of substance always contains the same number of particles (atoms, ions, molecules or formula units) irrespective of its state, solid, liquid or gaseous, that is 6.02x1023 particles. This constant number has been determined by several methods, called Avogadro's number (symbol NA),in the honor of Avogadro, the scientist who gave chemistry a method for finding atomic and molecular masses.


1 mole of C       =          12g       =          6.02x1023 atoms of carbon

1 mole of Mg     =          24g       =          6.02x1023 atoms of magnesium

1 mole of H2O    =          18g       =          6.02x1023 molecules of water

1 mole of CO2    =          44g       =          6.02x1023 molecules of O2

1 mole of NaCl   =          58.5g    =          6.02x1023 F-units of NaCl

1 mole of CaC03=          l00g      =          6.02x1023 F-units of CaCO3

1 mole of Na+   =          23g       =          6.02x1023 ions of Na+

1 mole of CI-     =          35.5g    =          6.02x1023 ions of CI-

 Conversion of Mass into Moles and Moles into Mass of Substance

Problem1. Calculate the number of moles, in 50 g of each.

(a) Na    (b) H2O


Number of moles    Given mass of substance

                            Atomic mass or Formula mass

(a)        Given,

i) Number of moles        =          ?

ii) Given mass of Na       =          50g

iii)    Atomic mass of Na =          23 a.m.u.

.'. Number of moles of Na = 50/33=2.173 moles of Na

 (b)        Given,

i) Number of moles of H2O          =          ?

ii) Formula mass of H2O =          18 a.m.u.

iii) Given mass of H2O                 =          50g

Number of moles of H2O = 50/18=2.777 moles of H2O

 Problem 2. What is the mass of 3 moles of each.

a) Al      b) CO2

 Number of moles =   Given mass of substance/Atomic mass or Formula mass

.'. Mass of substance = number of moles x atomic mass or formula mass in grams

 (a)        Given,

i) Number of moles of Al             =          3 moles

ii) Atomic mass of Al                   =          27 grams

iii| Mass al Al                              =          Mole x atomic mass of Al

                        = 3x 27=81g

(b)        Given,

i) Number of moles of CO2          =          3 moles

ii) Formula mass of CO2 =          44g

iii)  Mass of CO2                                                    =          Mole x Formula mass of CO2

                                                                        =          3x42=132

Use of Avogadro’s number:

a) To calculate the number of atoms or molecules in a given sample of substance.

b) To calculate the mass of single atom or molecule of any substance.


Problem: 1.

Calculate the number of atoms in 9g of Al.

Solution: According to Avogadro's number.

1 mole of Al= 27g = 6.02x1023 atoms.

This shows that:


27 g of Al contain      6.02x1023 atoms of Al.

1g of Al will contain 6.02x1023/27g

9g of Al will contain 6.02x1023 x9/27g        =2.006x1023

This numerical can also be solved, by using the formula.

Number of atoms          =NA x mass of substance/Atomic mass

Number of (Al) atoms    =6.02x1023 x9/27g

=2.006x1023 atoms of Al


Problem: 2.

Calculate the number of molecules in 9g of CO2.

Solution: According to Avogadro's number

1 mole of CO2 = 44g = 6.02x1023 molecules

This shows that:

44 g of CO2 contain 6.02x1023 molecules of CO2

1 g of CO2 contain 6.02x1023/44g molecules

9 g of CO2 will contain 6.02x1023x9g/44g

= 1.231x1023 molecules of CO2

By formula       

Number of molecules     =NA x mass of substance/Formula mass

Number of (CO2) molecules = 6.02x1023x9g/44g

=1.231x1023 molecules of CO2

Problem: 3.

Calculate the mass of one atom of carbon in grams

Solution: According to Avogadro's number

1 mole of C       = 12g = 6.02x1023 atoms

This indicates that

6.02x1023 atoms of C weigh 12 g

1 atoms of C     =12g/6.02x1023 = 12g x 10-23 /6.02

= 1.993 x 10-23g

Mass of one C-atom= 1.993 x 10-23g

By formula

Mass of one atom=Atomic mass in g/NA

Mass of one(C) atom=12g/6.02x1023 =1.993x02x1023g


Problem: 4.

Calculate the mass of molecule of water (H2O) in gram.


According to Avogadro’s number

1 mole of H2O = 18g = 6.02x1023 molecules

This indicates that

6.02x1023 molecules of (H2O) weight 18 g

1 molecules of (H2O) weight 18g/6.02x1023

= 18gx10-23/6.02=2.90x10-23g

By formula

Mass of one molecule = Formula mass in grams/NA

Mass of one (H2O) molecules = 18g/6.02x1023=2.90x10-23g

                        Mass of one (H2O) molecule in grams = 2.90x10-23g

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