The Mole & Avogadro’s Constant
The mole
- This is the mass of a substance containing the same number of fundamental units as there are atoms in exactly 12.000 g of 12C
- The mole is the unit representing the amount of atoms, ions, or molecules
- One mole is the amount of a substance that contains 6.02 x 1023 particles (Atoms, Molecules or Formulae) of a substance (6.02 x 1023 is known as the Avogadro Number)
Examples
- 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 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
Extended Only
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:
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 occupies
Equation:
Volume of gas (dm3) = Amount of gas (mol) x 24
or
Volume of gas (cm3) = Amount of gas (mol) x 24000
Example:
2. Calculating the moles in a particular volume of gas
Equation:
Amount of gas (mol) = Volume of gas (dm3) ÷ 24
or
Amount of gas (mol) = Volume of gas (cm3) ÷ 24000
Example:
