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?

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

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