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SAI International School
Class-IX

Subject- Chemistry
Topic-Atoms and Molecules
Topic-Mole concept

Lesson Note
Recaptulation:

Atomic mass or Relative atomic mass:Atomic mass of an element may be
th
defined as the average relative mass of an atom of the element as compared with 1/12
of the mass of an atom of carbon (C-12 isotope) taken as 12 amu.

Molecular mass or relative molecular mass of the substance:
The relative molecular mass of the substance can be defined as the mass of one
th
molecule of the substance compared with 1/12 of the mass of one atom of carbon
which is taken as a standard. It is expressed in terms of amu or u.

Gram Atomic Mass: The gram atomic mass of an element is its relative atomic
mass expressed in grams
e.g. Relative atomic mass of oxygen is 16 u and its Gram atomic mass is 16g
Gram Molecular Mass:The gram molecular mass of a substance is its relative
molecular mass expressed in grams.
E.g. Relative molecular mass of H 2O is 18 u and its gram atomic mass is 16 g

A mole is defined as the amount of a substance that contains exactly 6.022
23
X 10 ‘elementary entities’ of the given substance.
The number 6.022X10 is popularly known as the Avogadro constant and is often denoted by the
23
symbol ‘N A’. The elementary entities that can be represented in moles can be atoms, molecules,
monoatomic/polyatomic ions, and other particles (such as electrons).
For example, one mole of a pure carbon-12 ( C) sample will have a mass of exactly 12 grams and
12
will contain 6.022X10 (N A) number of C atoms. The number of moles of a substance in a given
23
12
pure sample can be represented by the following formula:
Mole Concept: A mole is defined as the amount of a substance that contains
exactly 6.02214076X10 ‘elementary entities’ of the given substance.
23
23
Mole: A collection of 6.022 × 10 particles is named as one mole.

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