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Lesson:Mole Concept

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The mole concept is an essential tool for the study of the fundamentals of chemical calculations. This concept is simple but its application in problems requires a thorough practice.  Do remember to subscribe to this lesson page to stay updated with new Question Bank every month.*

*The Question will start posting when a specific topic inside the lesson is completed.

In this chapter, we shall be discussing:

  • The Mole
  • Rules in Brief
  • Significance of Chemical Equations
  • Principle of Atom Conservation
  • Solved Examples

The Mole

The 14th General Conference on Weights and Measures(1971) adopted the mole(mol) as the basic SI unit of the amount of a substance.

There are many ways of measuring the amount of a substance, weight and volume being the most common. But the basic unit of chemistry is the atom or a molecule and to measure the number of atoms or molecules is, therefore, of foremost importance.

Mole in Latin means heap or mass or pile. A mole of atoms is a collection of atoms whose total weight is the number of grams equal to the atomic weight. As equal numbers of moles of different elements contain equal terms of moles. Just as a dozen means twelve objects, a score means twenty objects, chemists have defined a mole as a 'definite number' of particles, viz. atoms, molecules, ions or electrons, etc. This 'definite number' is called the Avogadro constant, equal to 6.022 * 10^23, in honour of Amedeo Avogadro. However, for many years scientists have made use of the concept of a mole without knowing the value of the Avogadro constant. Thus, amole of hydrogen atoms or a mole of hydrogen molecules or a mole of hydrogen ions or a mole of electrons means the Avogadro contant of hydrogen atoms, hydrogen molecules, hydrogen ions or electrons respectively.

The value of the Avogadro constant depends on the atomic-weight scale. At present the mole is defined as the amount of a substance containing as many atoms, molecules, ions, etc as there are carbon atoms in exactly 12 g of C-12 . The value of the Avogadro constant was changed by a very small amount in 1961 when the basis of the atomic-weight scale was changed from naturally occuring mixture of oxygen isotopes at 16 amu to C-12, whic put oxygen at 15.9994 amu.

It is quite interesting and surprising to know that the mole is such a big number that it will take 10^6 years to count just one mole at the rate of one count per second, and the world population would be only of the order of 10^-14 mole in chemical terminology.

Rules in Brief

The following are the definitions of 'mole' represented in the form of equations:

  1. Number of moles of molecules = weight(in g)/molecular weight
  2. Number of moles of atoms = weight(in g)/atomic weight
  3. Number of moles of gases = Volume(at NTP)/standard molar volume. Note: (Standard molar volume is the volume occupied by 1 mole of any gas at NTP, which is equal to 22.4 litres)
  4. Number of moles of atoms/molecules/ions/electrons = (number of atoms/molecules/ions/electrons)/Avogadro constant
  5. Number of moles of solute = molarity × volume of solution in litres or number of millimoles = molarity × volume of solution in mL
  6. For a compound MxNy, x moles of N = y moles of M

These rules are very important and have been frequently applied throughout the lesson.

Significance of Chemical Equations

A chemical equation describes the chemical process both qualitatively and quantitatively. The stoichiometric coefficients in the chemical equation give the quantitative information of the chemical process. These coefficients represent the relative number of molecules or moles of the reactants and products.


2.KCO3 (s)    → 2.KCl (s)    +    3.O2 (g)

2 molecules

2 N molecules

2 moles

2 molecules

2 N molecules

2 moles

    3 molecules

   3 N molecules

   3 moles

(N is Avogadro constant here)

Again, Avogadro's principle states that under the same conditions of molecules. Thus, for homogenous gaseous reactions, the stoichiometric coefficients of the chemical equation also signify the relative volumes of each reactant and product under the same conditions of temperature and pressure.



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