Fundamental Concepts of Chemistry -Mole Concept

9/24/2015 CBSE

Mole



Definition

It is defined as atomic mass of an element, molecular mass of a compound or
formula mass of a substance expressed in grams is called as mole.


OR

The amount of a substance that contains as many number of particles (atoms,
molecules or ions) as there are atoms contained in 12 gm of pure carbon.




Examples

�1. The atomic mass of hydrogen is one. If we take 1 gm of hydrogen, it is equal
to one mole of hydrogen.

�2. The atomic mass of Na is 23 if we take 23 gm of Na then it is equal to one
mole of Na.

�3. The atomic mass of sulphur is 32. When we take 32 gm of sulphur then it is
called one mole of sulphur.

�From these examples we can say that atomic mass of an element expressed in grams
is called mole.

�Similarly molecular masses expressed in grams is also known as mole e.g.

�The molecular mass of CO2 is 44. If we take 44 gm of CO2 it is called one mole
of CO2 or the molecular mass of H2O is 18. If we take 18 gm of H2O it is called
one mole of H2O.

�When atomic mass of an element expressed in grams it is called gram atom


�While

�The molecular mass of a compound expressed in grams is called gram molecule.


�According to the definition of mole.

�One gram atom contain 6.02 x 1023 atoms

�While

�One gram molecule contain 6.02 x 1023 molecules.





Avagadro's Number



�An Italian scientist, Avagadro's calculated that the number of particles (atoms,
molecules) in one mole of a substance are always equal to 6.02 x 10(23). This
number is known as Avogadro's number and
represented
as N(A).



Example

�1 gm mole of Na contain 6.02 x 1023 atoms of Na.

�1 gm mole of Sulphur = 6.02 x 1023 atoms of Sulphur.

�1 gm mole of H2SO4 = 6.02 x 1023 molecules H2SO4

�1 gm mole of H2O = 6.02 x 1023 molecules of H2O

�On the basis of Avogadro's Number "mole" is also defined as

Mass of 6.02 x 1023 molecules, atoms or ions in gram is called mole.




Determination Of The Number Of Atoms Or Molecules In The Given Mass Of A
Substance




Example 1

�Calculate the number of atoms in 9.2 gm of Na.



Solution

�Atomic mass of Na = 23 a.m.u

�If we take 23 gm of Na, it is equal to 1 mole.

�23 gm of Na contain 6.02 x 1023 atoms

�1 gm of Na contain 6.02 x 1023 / 23 atoms

�9.2 gm of Na contain 9.2 x 6.02 x 1023 /23

�= 2.408 x 1023 atoms of Na



Determination Of The Mass Of Given Number Of Atoms Or Molecules Of A
Substance




Example 2

�Calculate the mass in grams of 3.01 x 1023 molecules of glucose.



Solution

�Molecular mass of glucose = 180 a.m.u

�So when we take 180 gm of glucose it is equal to one mole So,

�6.02 x 1023 molecules of glucose = 180 gm

�1 molecule of glucose = 180 / 6.02 x 1023 gm

�3.01 x 1023 molecules of glucose = 3.01 x 1023 x 180
/ 6.02 x 1023

�= 90 gm


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