Mole Concept

Mole concept is a fundamental principle in Chemistry that helps chemists quantify the amount of substances.

The nature of matter encompasses its fundamental properties and behaviors, explored through Chemistry. Matter is composed of atoms and molecules, which interact and combine to form various substances with distinct characteristics.

1.0What is Mole Concept

To understand the Mole concept, first we need to know the difference between Relative mass and Molar mass.

Relative Atomic Weight is a pure number, without any unit.

Example: Na = 23; H = 1; O = 16; N = 14, S = 32, Fe = 56, K = 39, C = 12

The quantity 1/12^th mass of an atom of C¹² is known as one atomic mass unit (amu).

1 atomic mass unit (amu) = 1.67× 10^–24 gram

                              = 1.67× 10^–27 Kg

Note: Actual mass of one atom of element = Atomic weight X amu

Example:  Actual mass of one atom of Oxygen  = 16 x amu or 16 amu.

                                                                            = 16 x 1.67× 10^–24 g

           = 26.72 × 10^–24 g

Molecular Weight-

Relative molecular weight is also a pure number, without any unit.

Example: H₂O = 18; NH₃ = 17; H₂SO₄ = 98; SO₂ = 64

Note: Actual mass of one molecule of compound = Molecular weight x amu

Example: Actual mass of one molecule of Ammonia  = 17 x amu or 17amu.

                                                                                    = 17 x 1.67× 10^–24 gram

                                                                                    = 28.39 x 10^–24 gram

2.0What is a Mole?

A mole (mol) is a unit used to express the amount of a chemical substance. It represents a specific number of atoms, ions, molecules, or formula units depending on the substance. Let’s understand how a mole is related to atoms and atomic weight. 

Collectively atoms, molecules, ions, electrons, protons, neutrons etc. are called Fundamental Particles.

3.0Atoms and Atomic Weight

Atoms are the fundamental building blocks of matter, composed of a nucleus containing protons and neutrons, surrounded by electrons in orbitals. Each element is defined by the number of protons in its nucleus, known as the atomic number, which determines its unique chemical identity.

Atomic weight (Relative Atomic weight) =  $\frac{\text{ Actual mass of one atom of element }}{1/12\text{ th mass of one atom of Carbon }-12}$

$\frac{\text{ Mass of }1\text{ atom of an element }}{\frac{1}{12}\times \text{ Mass of }1\text{ atom of }\mathrm{C}-12}$

Basic definition of mole concept

Mole Concept is a method used to quantify the amount of substance. It's experimentally proven that one gram atom of any element, or one gram molecule of any substance, contains the same number of entities, known as Avogadro's number: approximately 6.022 × 10²³ (denoted by N_A).

The determination of this number was based on the mass of a carbon-12 atom, which was measured to be 1.992648×10⁻²³ grams. Therefore, one mole of carbon-12, weighing 12 grams, contains 6.022×10²³ atoms. 

This number remains consistent for all substances, regardless of their identity.

Gram Atomic Mass

Gram Atomic Mass of an element is defined as the mass of one mole of atoms of that element, where the mass is numerically equivalent to the element's atomic mass expressed in grams. This measurement indicates how much one mole of the element weighs and is equal to the atomic mass unit (amu) value of the element when expressed. 

Gram atomic mass = mass of 1 gram atom = mass of 1 mole atom

  = mass of N_A atoms = mass of 6.023 × 10²³ atoms.

Example:

Gram atomic mass of oxygen = mass of 1 g atom of oxygen = mass of 1 mole atom of oxygen.

                                                                                           = mass of N_A atoms of oxygen

                                                                                           =  (16/ N_A) X N_A = 16 g

Gram Molecular Mass

Gram Molecular Mass of a substance is defined as the mass of one mole of its molecules when the molecular mass is expressed in grams. This value quantifies the weight of one mole of the substance and corresponds to the numerical value of its molecular mass.

Gram molecular mass  = mass of 1 gram molecule = mass of 1 mole molecule

                                     = mass of N_A molecules = mass of 6.023 × 10²³ molecules

Example:

Gram molecular mass of H₂SO₄ = mass of 1 gram molecule of H₂SO₄

                                                             = mass of 1 mole molecule of H₂SO₄

                                                             = mass of N_A molecules of H₂SO₄

                                                       =  (98/N_A)X N_A = 98 g

4.0Mole Concept Basic Formulas

5.0General Basic Questions on Mole Concept with Solutions

Q.1 Find the total number of H, S and 'O' atoms in the following:

(1) 196 g H₂SO₄                  

(2) 196 amu H₂SO₄    

(3) 5 mole H₂S₂O₈      

(4) 3 molecules H₂S₂O₆

Solution:

(1) H = 4N_A,             S = 2N_A,                  O = 8N_A atoms 

(2) H  = 4 atoms,    S = 2 atoms,         O = 8 atoms.

(3)  H = 10N_A,         S = 10N_A,               O = 40 N_A atoms

(4)  H = 6 atoms,    S = 6 atoms,         O = 18 atoms.

Q.2 Calculate the number of moles in 11.2 liters of Oxygen at STP. 

Solution:

∵ At STP, 22.4 litre of Oxygen contains = 1 mol

∴ At STP, 11.2 litre of Oxygen contains =  1/ 22.4 × 11.2 = 0.5 mol

OR

Mole of gas =  V/ 22.4 (at NTP/STP)

      =  11.2/22.4  = 0.5 mol