Avogadro’s Number and Mole – How is Mole Calculated

Mole is a counting device or a number like:

12 things   =  Dozen

144 things  =  Gross

2 things = Pair

1000 g    =   1k g

similarly:

6.022 x 1023 particles =1 mole

Mole Definition:

In Chemistry , mole is defined as the amount of substance that exactly contains 6.022 x 1023 particles. .

For Example:

12g C-atom  = 1 mole carbon = 6.022x 1023 C -atoms

 18g H2O molecule   = 1 mole carbon = 6.022x 1023 H2O molecule

 58.5g NaCl Formula unit = = 1 mole carbon = 6.022x 1023 NaCl Formula unit

17 g OH-1 ions = 1 mole carbon = 6.022x 1023 OH-1 ions

so the mole is also defined as:

Atomic mass, molecular mass and formula mass when expressed in grams is equal to one mole.

e.g.

Atomic mass of carbon atom   =  12 amu

Atomic mass when expressed in grams   =12g= 1 mole

Molecular mass of water molecule = 18 amu

Molecular mass when expressed in grams   =18g= 1 mole

Formula mass of NaCl   =   58.5amu

Formula mass when expressed in grams =   58.5g=1mole

Avogadro’s number:

Avogadro’s number is defined as the number Of 6.022 x 1023 particles present in one mole of a substance.

Avogadro’s number is denoted by NA .

NA = 6.022 x 1023 particles =1 mole

Formulas to calculate number of moles and Avogadros number of particles:

No of moles  =       mass

                        molar mass


Number of moles   =      Number of particles      

         6.022 x 1023


No of moles of gas at STP = volume of gas at STP

                                                   molar Volume


No of particles = Moles  X  Avogadro’s number


Mass of a substance = no of moles  x  Molar mass

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