Mole Concept Explained: Class 10 Science Notes on Atoms and Molecules

Mole Concept

Introduction to the Mole Concept

So far we discussed about matters in terms of individual atoms and molecules. Atomic mass units provide a relative scale for the masses­ of the elements. Since the atoms have such small masses, no usable scale can be devised to weigh them in the calibrated units of atomic mass units. In any real situation, we deal with ­macroscopic samples containing enormous number of atoms­. Therefore, it is convenient to have a special unit to describe a very large number of atoms. The idea of a ‘unit’ to denote a ­particular number of objects is not new. For example, the pair (2 items) and the dozen (12 items), are all familiar units. Chemists measure atoms and molecules in ‘moles’. So, you can now understand that ‘mole’ denotes a number of particles.

In the SI system, the mole (mol) is the amount of a substance that contains as many elementary entities (atoms, molecules, or other particles) as there are atoms in exactly 12 g (or 0.012 kg) of the carbon-12 isotope. The actual number of atoms in 12 g of carbon-12 is determined experimentally. This is called Avogadro’s Number (N_A), named after an Italian scientist Amedeo Avogadro who proposed its significance. Its value is 6.023 × 10²³. So one mole of a substance contains 6.023 × 10²³ entities. Thus, 5 moles of oxygen molecules contain 5 × 6.023 ×10²³ molecules.

Mole Concept: The study of the collection of particles by using mole as the counting unit, in order to express the mass and volume of such unit particles in a bulk of matter is known as mole concept.

Calculating the Number of Moles

The number of moles of a substance can be calculated by various means depending on the data available, as follows:

• Number of moles of molecules.
• Number of moles of atoms.
• Number of moles of a gas (Standard molar volume at STP = 22.4 litre).
• Number of moles of ions.

Figure 7.3: Mole Concept Relationships

Note: STP-Standard Temperature and Pressure (273.15 K, 1.00 atm)

Mole of atoms:

One mole of an element contains 6.023 × 10²³ atoms and it is equal to its gram atomic mass.

i.e., one mole of oxygen contains 6.023 × 10²³ atoms of oxygen and its gram atomic mass is 16 g.

Mole of molecules:

One mole of matter contains 6.023 × 10²³ molecules and it is equal to its gram molecular mass.

i.e., one mole of oxygen contains 6.023 × 10²³ molecules of oxygen and its gram molecular mass is 32 g.

Molar volume:

One mole of any gas occupies 22.4 litre or 22400 ml at S.T.P. This volume is called as molar volume.

Calculation of number of moles by Different modes

Number of moles = Mass / Atomic Mass

Number of moles = Mass / Molecular mass

Number of moles = Number of Atoms / 6.023 × 10²³

Number of moles = Number of Molecules / 6.023 × 10²³

Figure 7.3: Mole Concept Summary