Amount of Substance (OCR A Level Chemistry A): Revision Note

The mole & the Avogadro constant

Amount of substance and the mole

• Amount of substance refers to the quantity used to count particles in a substance

  • It is represented by the symbol n in equations

  • The unit of amount of substance is the mole (mol)

The Avogadro constant

• Amount of substance links to the Avogadro constant, N_A,

  • The Avogadro constant defines the number of particles in one mole of a substance

  • This constant applies to atoms, molecules, ions and electrons

  • The value of N_A is 6.02 x 10²³ mol^-1 

Molar mass

• The mass of a substance that contains this number of particles is called one mole

  • This mass is known as the molar mass

  • It is defined as the mass of substance that contains the same number of particles as there are in exactly 12.00 g of carbon-12

• The amount of substance (n), mass (m), and molar mass (M) are related by the equation:

n =

Molar gas volume

• Molar gas volume is the volume occupied by one mole of any gas at room temperature and pressure (RTP)

  • At RTP, molar gas volume is 24.0 dm³ mol^-1

• One mole of an element has a mass equal to its relative atomic mass in grams:

  • One mole of carbon contains 6.02 × 10²³ atoms and has a mass of 12.0 g

  • One mole of water contains 6.02 × 10²³ molecules and has a mass of 18.0 g

    • (2 hydrogen atoms = 2 × 1.0, 1 oxygen atom = 16.0, total = 18.0 g)