Structure 1.4 – Counting particles by mass : The mole

S1.4.1 – The mole as the unit of amount

📌 Avogadro constant

  • The Avogadro constant is a relative amount that defines the mole as a unit of amount
  • Avogadro’s number is 6.02 x 1023 and is known as the ‘mole’ (SI unit : mol)
  • One mole of anything consists of 6.02 x 1023 entities (atoms, molecules, ions etc)

How to calculate moles using Avogadro’s constant

NUMBER OF MOLES = NUMBER OF PARTICLES / AVOGADRO’S CONSTANT

Example

QUESTION : A solution of ammonia and water contains 2.10 × 1023 molecules of H20 and
8.00 × 1021 molecules of NH. How many moles of hydrogen atoms are present?

SOLUTION : First total the number of hydrogen atoms.
from water, H2O: number of H atoms = 2 × (2.10 × 1023) = 4.20 × 1023
from ammonia, NH;: number of H atoms = 3 × (8.00 × 1021) = 0.240 × 1023
so total H atoms = (4.20 × 1023) + (0.240 × 1023) = 4.44 × 1023
To convert atoms to moles, divide by the Avogadro constant:

Total moles : (4.44 × 1023 / 6.02 × 1023) = 0.738 mols