Question

What is Mole Concept in class 9

Answer 1

Mole: One mole is defined as the amount of substance of a system which contains as many entities like, atoms, molecules and ions as there are atoms in 12 grams of carbon - 12".

Later it was found that one gram atom of any substance contains as many atoms as are present in 1 gram atom of  carbon¹²isotope.
The value of this number is found to be 6.023X10²³ and this is called as Avogadro's number.

Avogadro number:  The number of the particles present in one mole of any substance is equal to 6.022x10²³. This is called avogadro’s number or avogadro’s constant. 
It is represented by N.

Number of particles in 1 mole: 
1 mole of hydrogen atoms represents 6.022 × 10²³ hydrogen atoms.
1 mole of hydrogen molecules represents 6.022 × 10²³ hydrogen molecules.
1 mole of water molecules represents 6.022 × 10²³ water molecules.

Importance of mole concept and Avogadro's number
mole concept and Avogadro's number help in the calculations in so many ways. They are,

i) Helps in the calculation of actual mass of single atom of an element or single molecule of a substance
Examples
a) Find the mass of 1 atom of nitrogen
Solution - 1 mole of nitrogen = 14g = 6.023x10²³ atoms
Mass of  6.023x10²³ atoms = 14g
Mass of 1 atom of nitrogen = 14 /6.023x10²³ 
= 2.32 X 10^-23 g.
b) Calculate the mass of a molecule of CO
1 mole of CO= 28g
1 mole CO =  6.023x10²³ molecules
Therefore mass of 6.023x10²³ molecules of CO = 28g
Mass of 1 molecule of CO = 28/6.023x10²³
= 4.64 X10^-23  g

ii) Helps in the calculation ofnumber of atoms or molecules present in a given mass of element or compound.
Example

Calculate the number of water molecules contained in a drop of water weighing 0.06g ?

Ans: 18g of water = 1 mole
0.06 g = ?
= 0.06/18 = 3.33 X 10^-3 mole of water
1 mole of water contains - 6.022X10²³ molecules of water
3.33 X 10^-3 mole of water contains -----?
= 6.022X10²³ X 3.33 X 10^-3
= 2 X 10²⁰ molecule of water
∴ 0.06g of water contains 2 X 10²⁰ molecules of water.

iii) Helps in the calculation of number of molecules present in a given volume of the gas under given conditions.
Example
Calculate the number of molecules present in 600c.c of NH₃ at S.T.P.
Hint - This problem can be solved by using mole concept.
Solution - 1 mole of NH₃ = 6.023 X 10²³ molecules = 22400 c.c at STP
That is 22400 c.c of NH₃ at STP contains  6.023 X 10²³ molecules 
600 c.c of NH₃ at STP contains = 6.023 x10²³/22400 X 600  = 0.161 X 10²³ molecules

Answer 2

It is the concept in chemistry, which is used for calculating quantitative problems . for ex

if we heat 120 gm of CaCO3 then how much CO2 is obtained ? Mole concept is used to solve this type of problems mainly