AVOGADRO’s NUMBER AND MOLE

Avogadro’s  Number

                            In chemistry, we deal with substances that are composed of atoms, molecules, and formula units. The counting of these parties 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 × 10^23 particles. It is represented by ‘N_A’. Hence, the 6.02 × 10^23 numbers of atoms, molecules, and formula units are called Avogadro’s number that is equivalent to one ‘mole’ of respective substance. In simple words, 6.02 × 10^23 particles are equal to one mole as twelve eggs are equal to dozen.

To understand the relationship between the Avogadro’s number and the mole of a substance let us consider a few examples.

     ^i.                        6.02 × 10^23 atoms of carbon are equivalent to one mole of carbon.

ii.            6.02 × 10^23 molecules of water are equivalent to one mole of water.

iii.            6.02 × 10^23 formula units of sodium chloride are equivalent to one mole of sodium chloride.

Thus 6.02 × 10^23 atoms of elements or 6.02 × 10^23 molecules of molecular substance or 6.02 × 10^23               formula units of an ionic compound are equivalent to 1 mole.

                                          For further explanation about the number of atoms in molecular compounds let us discuss two examples:

 i.            One the molecule of water is made up of 2 atoms of hydrogen and 1 atom of oxygen, hence 2×6.02 × 10^23 atoms of hydrogen and 6.02 × 10^23 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 × 10^23 numbers of Na⁺ and 6.02 × 10^23 Cl^- ions in one mole of sodium chloride. Thus, the total number of ions in 1 mole of NaCl in 12.04 × 10^24.

MOLE (chemist secret unit):

                                                       A mole is defined as the amount of a substance that contains 6.02 × 10^23 number of particles (atoms, molecules or formula units). It establishes a link between the mass of a substance and the number of particles as shown in the summary of molar calculations. It is abbreviated as ‘mol’.

                                                        You know that a substance may be an element or compound (molecular or ionic). The 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 mass.

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

For Example:

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

Molecular mass of H₂O expressed as 18g       = 1-mole o water

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

The formula mass of NaCl expressed as 58.5g        =1 mole of NaCl

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

Number of moles            = known mass of a substance ÷ molar mass of the substance

Or,

    Mass of a substance (g)       =     numbers of moles × molar mass

 

 

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