Concept of Mole

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Card 1: Define mole?


A mole is defined as the amount of substance which contains the Avogadro Number of particles.

Card 2: State the meaning of Avogadro Constant.

Avogadro Constant

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.

Card 3: What is the meaning of molar mass.

Molar Mass

  1. Molar mass is the mass of 1 mol of substance.
  2. The unit of molar mass is g/mol.
  3. The molar mass of a substance is always equal to its relative atomic mass or relative molecular mass in the unit of g/mol.

Relative atomic mass of oxygen atom = 16, hence the molar mass of oxygen atom = 16g/mol

Relative moleculoar mass of oxygen molecule (O2) = 32, hence the molar mass of oxygen molecule is 32g/mol

Card 4: Interconversion of number of moles and number of particles.

Number of Moles ↔ Number of Particles

Example 1
Find the number of molecules in 2.5 mol of ammonia.
(Avogadro constant = 6.02 x 1023)


Number of molecules = 2.5 x (6.02 x 1023) = 1.505 x 1024 molecules.

Example 2

Calculate the mole of ammonia that contain 1.806 x 1021 ammonia molecule.


Number of mole = 1.806 x 1021 ÷ 6.02 x 1023 = 0.003 mol

Card 5: Interconversion of number of moles and mass of substance.

Number of Moles ↔ Mass of Substance

Mole and Mass

Example 3
Find the mass of 2.5 mol ammonia (NH3).
(Relative atomic mass: N = 14; H = 1)


Relative molecular mass of ammonia (NH3) = 14 + 3(1) = 17.
Therefore, the molar mass of ammonia = 17g/mol

Mass of 2.5 mol ammonia = 2.5 x 17 = 42.5g

Example 4
How many moles of atom exist in 69g of sodium metal?
(Relative Atomic Mass: Sodium = 23)


Molar mass of sodium = 23g/mol

Number of moles of sodium = 69 ÷ 23 = 3 mol.

Card 6: State the meaning of molar volume of gas.

Molar Volume of Gas

Molar volume of gas is the volume of 1 mole gas.

One mol of any gas occupies a volume of 24 dm3 (24 000 cm3 ) at room temperature(25oC) and pressure or 22.4 dm3 (22400cm3) at s.t.p. (Standard Temperature(0oC) and Pressure).

Card 7: Interconversion of number of moles and volume of gas.

Number of Moles ↔ Volume of Gas

Mole and Volume of Gass

Example 5
Find the number of moles of nitrogen molecule in 1200cm3 of nitrogen gas at room temperature. (Molar volmue of gas at room temperature = 24dm3/mol)


1 mol gas ≡ 24 dm3 = 24000cm3

Number of moles of nitrogen = 1200 ÷ 24000 = 0.05 mol

Example 6
Find the volume of 2 mol nitrogen gas at s.t.p.
(Molar volmue of gas at s.t.p. = 22.4dm3/mol)


Volume of nitrogen gas = 2 x 22.4 = 44.8 dm3


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