The Mole & Avogadro’s Constant

The Mole Concept

Mole: This is the mass of any substance containing the same number of fundamental units i.e.; Atoms, Molecules or ions, as there are atoms in exactly 12.000 g of ¹²C

The mole is a unit representing the amount of Atoms, Molecules or Ions.

One mole is the amount of a substance that contains 6.02 x 10²³ particles (Atoms, Molecules or ions) of a substance (6.02 x 10²³ is known as the Avogadro Number)

Some Examples of mole amount

1 mole of Sodium (Na) contains 6.02 x 1023 Atoms of Sodium

1 mole of Hydrogen (H2) contains 6.02 x 1023 Molecules of Hydrogen

1 mole of Sodium Chloride (NaCl) contains  6.02 x 1023 Formula units of Sodium Chloride

Linking the mole and the atomic mass

One mole of any element is equal to the relative atomic mass of that element in grams.

For example one mole of carbon, that is if you had 6.02 x 10²³ atoms of carbon in your hand, it would have a mass of 12g, So one mole of helium atoms would have a mass of 4g, lithium 7g etc. For a compound we add up the relative atomic masses

So one mole of water (which have one oxygen and 2 hydrogens) would have a mass of 2 x 1 + 16 = 18g

Hydrogen which has an atomic mass of 1 is therefore equal to ¹/₁₂ the mass of a ¹²C atom, So one carbon atom has the same mass as 12 hydrogen atoms

The Mole & the Volume of Gases

Molar volume

This is the volume that one mole of any gas (be it molecular such as CO₂ or monoatomic such as helium) will occupy.

It’s value is 24dm³ or 24,000 cm³ at room temperature and pressure (R.T.P.)

Calculations involving gases

General equation used,

Amount of gas (mol) = Volume of gas (dm3) ÷ 24

or

Amount of gas (mol) = Volume of gas (cm3) ÷ 24000

1. Calculating the volume of gas that a particular amount of moles occupiesEquation:

Volume of gas (dm3) = Amount of gas (mol) x 24

or

Volume of gas (cm3) = Amount of gas (mol) x 24000