The Mole Concept


 
 
Concept Explanation
 

The Mole Concept

Mole Concept:

Wilhelm Ostwald introduced the term ‘mole’ in 1896, which was accepted as unit to represent a large quantity of atoms in 1967. The mole is the amount of a substance which contains as many particles (atoms, ions, molecules, formula unit etc.) as in 12gm of C-12. Thus, one mole of any species is that quantity in number having a mass equal to its atomic or molecular mass in grams.

The number of particles present in 1 mole of any substance is same and fixed which is equal to 6.022 X 10^{23}. This is a constant, known as Avogadro constant or Avogadro number (N_A) named in the honour of Italian scientist Amedeo Avogadro. Thus, mole is also defined as number of particles equal to the Avogadro constant.

1 mole =6.022 X 10^{23}particles.

Derivation:

We know that mass of 1 atom is measured in terms of united mass (u) 1u=1.6605;X;10^{-24} g. So, 1g of the element contains frac{1}{1.6605;X;10^{-24}}  atoms of the element. It comes out to be approximately 6.022times10^{23}atoms.

Illustration: How many gms of each of the following elements must be taken to get 1 mole of the element?

1. Sodium     2. Chlorine     3.  Copper

Solution: The mass of 1 mole of an element is its atomic mass expressed in grams. Since the atomic masses of sodium, chlorine and copper are 23g, 35.5g and 63.5g respectively, we get 

1. sodium =23g

2. chlorine = 35.5g

3. copper = 63.5g

Illustration: Calculate the number of moles in 5.75g of sodium. (Atomic mass of sodium = 23)

Solution: 1 mole of sodium atoms = gram atomic mass of sodium = 23g

                  23g of sodium = 1 mole of sodium.

              5.75g; of; sodium  = frac{5.75}{23}:mol: of: sodium = 0.25: mole

Sample Questions
(More Questions for each concept available in Login)
Question : 1

________________________ named in the honour of Italian scientist Amedeo Avogadro.

Right Option : D
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Explanation
Question : 2

How many gms of element 'Copper' must be taken to get 1 mole of the element 'Copper' ?

Right Option : D
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Explanation
Question : 3

The number of particles present in 1 mole of any substance is same and fixed which is equal to ___________________________.

Right Option : C
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Explanation
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