Concept of Mole

  1. A mole is defined as the amount of substance which contains the Avogadro Number of particles.
  2. The Avogadro Number (or Avogadro Constant) is defined as the number of atoms in 12 g of the carbon-12 isotope, which is equal to 6.02 x 1023.
1 mole = 6.02 x 1023

Example:
1 mol of atoms = 6.02 x 1023 atoms
½ mol of atoms = 3.01 x 1023 atoms
¼ mol of atoms = 1.505 x 1023 atoms.

Note:
The particles in a material can be atoms, molecules or ions.
Therefore
1 mol potassium atom = 6.02 x 1023 potassium atom
1 mol carbon dioxide molecules = 6.02 x 1023 of carbon dioxide molecules.
1 mol sulphate ions = 6.02 x 1023 sulphate ions.

Example:
Find the number of atoms in:
  1. 2 mol ferum 
  2. 3.6 mol zink 
  3. 2.8 mol zink 
  4. ¼ mol ferum 
Answer:
  1. 2 mol iron = 2 x 6.02 x 1023 = 1.204 x 1024  iron atoms
  2. 3.6 mol zink =   3.6 x 6.02 x 1023 = 2.167 x 1024  zink atoms
  3. 2.8 mol zink = 2.8 x 6.02 x 1023 = 1.686 x 1024 zink atoms
  4. ¼ mol iron = ¼ x 6.02 x 1023 = 1.505 x 1023  iron atoms.

Mole and mol

  1. Mole is the unit of amount of substance.
  2. “mol” is the symbol of mole.

Number of Mole and Number of Particles


  1. We have just learn that, mole is a quantity, and it is equal to 6.02 x 1023. The number 6.02 x 1023 is called the Avogadro constant.  
  2. Therefore, if we are given the number of mole of substance, and asked to find the number of particles (atoms, molecules or ions) in it, we multiply the number of mole by the Avogadro constant. 
  3. Likewise, if we are given the number of particles, and asked to find the number of mole of the particles, we divide the number of particles by the Avogadro constant.


Example
Which contains more atoms, 1 mol of helium or 1 mol of uranium? Which has a greater mass? [ RAM: He=4; U=238 ]

Answer:
1 mol of helium and 1 mol of uranium has equal number of atoms.

The mass of one Uranium atom ss greater than the mass of one helium atom.

Example
Find the number of atoms in 2.5 mol of gold.

Answer:
Number of atoms
= Number of mole x Avogadro constant
= 2.5 x 6.02 x 1023 = 1.505 x 1024 

Example
How many moles of magnesium that contain 2.76 x 1023 of magnesium atom?

Answer:
Number of mole
= Number of atoms ÷ Avogadro constant
= 2.76 x 1023 ÷ 6.02 x 1023
= 0.46 mol

Number of Mole of Atoms and Number of Mole of Molecules

  1. All molecules contain more than one atom.
  2. For example, in a carbon dioxide molecule (CO2), there are 3 atoms – 1 carbon atom and 2 oxygen atoms.
  3. Therefore, in 5 carbon dioxide molecules, there will be 15 atoms  – 5 carbon atom and 10 oxygen atoms.
  4. Similarly,  in 1 mole carbon dioxide molecules, there will be 3 mole atoms  – 1 mole carbon atoms and 2 mole oxygen atoms, and in 3 mole carbon dioxide molecules, there will be 9 mole atoms  – 3 mole carbon atoms and 6 mole oxygen atoms, so on and so forth.

Example:

  1. How many hydrogen molecules can be made with 20 mol of hydrogen atoms?
  2. How many hydrogen atoms are there in 20 mol of hydrogen molecules?

Answer:
a. 
Each hydrogen molecules (H2) consist of 2 hydrogen atoms.
Therefore, 20 mol of hydrogen atoms can make 10 mole of hydrogen molecules.

b. Number of mole of hydrogen atoms in 20 mole of hydrogen molecules

= 2 x 20 mole = 40 mole.

Example:

  1. Find the number of carbon atom in 0.75 mol of carbon dioxide.
  2. Find the number of oxygen atom in 0.75 mol of carbon dioxide.

Answer:
a. 
Each carbon dioxide molecules (CO2) consist of 1 carbon atom and 2 oxygen molecules.
Therefore, 0.75 mole of carbon dioxide contain 0.75 mole carbon atoms.

b.

Number of mole of oxygen atoms in 0.75 mole of carbon dioxide
= 2 x 0.75 mole = 1.50 moles.