The word mole means pile up .
This concept originated from a simple problem that was faced by the scientists to calculate the reactions. they needed a concept that could relate number of molecules with the weight.
A mole of a substance is equal to as many molecules of that substance as there are number of atoms in exactly 12 gram of carbon C-12 isotope .
As in case of dozen we have 12 items of any substance
As in century we have 100 nubers
Similarly in case of MOLE we have 6.022 X 1023 of any substance either atoms or molecules. this is also called the avogadro number.
one mole is also equal to the molecular mass or atomic mass of a substance expressed in grams. for example:-
12 grams of carbon(atomic mass 12 amu) is equal to 1 mole.
1 gram of Hydrogen(atomic mass 1amu) is equal to 1 mole.
18 grams of water, H2O (molecular mass 18 amu) is equal to 1 mole.
GRAM ATOMIC MASS OR GRAM MOLECULAR MASS : The molecular mass or atomic mass expressed in grams .
And the gram molecular mass is also called the molar mass.
How we can calculate the number of moles?
Here is the answer
number of moles = given mass/ molar mass
Lets take an example to find number of moles
we are provided with 18 grams of helium , and the atomic mass of helium is 4 amu now lets calculate the number of moles.
Given mass = 18g
Molar mass = 4 g
Number of moles = given mass / molar mass
Number of moles = 18 / 4
Number of moles = 4.5
and if you are intrested to calculate the number of particles (either atoms or molecules), then simply multiply the number of moles to Avogadro Number.
number of atoms of helium = number of moles X Avogadro number
number of atoms in 18 grams or 4.5 moles of helium = 4.5 X 6.022 X 1023
the number of atoms are = 27.099 X 1023