1.5 Avogadro’s Number and Mole

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Avogadro's Number

In chemistry we deal with substances which are composed of atoms, molecules or formula units. The counting of these particles is not possible for the chemists. The concept of Avogadro's number facilitated the counting of particles contained in the given mass of a substance.

Avogadro's Number is a collection of 6.02 X 1023 particles. It is represented by symbol 'NA'. Hence, the 6.02 X 1023 number of atoms, molecules or formula units is called Avogadro's number that is equivalent to one 'mole' of respective substance. In simple words, 6.02 X 1023 particles are equal to one mole as twelve eggs are equal to one dozen.

To understand the relationship between the Avogadro's number and the mole of a substance let us consider a few examples.
  • 6.02 X 1023 atoms of carbon are equivalent to one mole of carbon.
  • 6.02 X 1023 molecules of H2O are equivalent to one mole of water.
6.02 X 1023 formula units of NaCl are equivalent to one mole of sodium chloride.

Thus, 6.02 X 1023 atoms of elements or 6.02 X 1023 molecules of molecular substance or 6.02 X 1023 formula units of ionic compounds are equivalent to 1 mole.
For further explanation about number of atoms in molecular compounds or number of ions in ionic compounds let us discuss two examples:

  • One molecule of water is made up of 2 atoms of hydrogen and 1 atom of oxygen, hence 2 X 6.02 X 1023 atoms of hydrogen and 6.02 X 1023 atoms of oxygen constitute one mole of water.
  • One formula unit of sodium chloride consists of one sodium ion and one chloride ion. So there are 6.02 X 1023 number of Na ions and 6.02 X 1023 CI ions in one mole of sodium chloride. Thus, the total number of ions in 1 mole of NaCl is 12.04 X 1023 or 24 1.204 X 1024 .

Mole (Chemist secret unit)

A mole is defined as the amount(mass) of a substance that contains 6.02 X 1023 number of particles (atoms, molecules or formula units). It establishes a link between mass of a substance and number of particles as shown in summary of molar calculations. It is abbreviated as 'mol'.

You know that a substance may be an element or compound (molecular or ionic). Mass of a substance is either one of the following: atomic mass, molecular mass or formula mass. These masses are expressed in atomic mass units (amu). But when these masses are expressed in grams, they are called as molar masses.

Scientists have agreed that Avogadro's number of particles are present in one molar mass of a substance. Thus, quantitative definition of mole is the atomic mass, molecular mass or formula mass of a substance expressed in grams is called mole.

For Example:

Atomic mass of carbon expressed as 12 g = 1 mol of carbon

Molecular mass of H2O expressed as 18 g = 1 mol of water

Molecular mass of H2SO4 expressed as 98 g = 1 mol of H2SO4

Formula mass of NaCl expressed as 58.5 g = 1 mol of NaCl

Thus, the relationship between mole and mass can be expressed as:

Number of moles (mol)    =   known mass of a substance / molar mass of the substance
Mass of substance (g)       =   number of moles x molar mass 

A detailed relationship between a substance and a mole through molar mass and number of particles is presented here.
Summary showing a relationship between a substance and a mole.


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Hello, this is Misbah ud Din a student of STEM. This is my blog chemisTREEpk which provides a vast knowledge about Chemistry(and sometimes other sciences) fully free.

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