The mole - Higher
Introducing the mole, the unit of measurement for the number of particles in a substance
It is often useful in chemical calculations to know the number of atomThe smallest part of an element that can exist. or molecules present. The moleThe amount of substance that contains the same number of particles as there are atoms in 12 g of carbon-12 (contains the Avogadro's constant 6.0 ×10²³ number of particles)., mol, is the unit for the amount of substance.
Avogadro constant
One mole contains the same number of particles as there are atoms in 12 g of carbon-12. As atoms and molecules are so small, the number of particles in a mole is very large. This number is:
- called the Avogadro constantThe number of particles in one mole of particles, 6.02 × 10²³ per mole.
- equal to 60,000,000,000,000,000,000,000 (6.0 × 1023) atoms, molecules or formula units
Calculating the number of particles
The number of particles of a substance can be calculated using:
- the Avogadro constant, 6.0 × 1023
- the number of moles
number of particles = 6.0 × 1023 x number of moles
Example
Calculate the number of water molecules in 0.5 mol of water.
Number of water molecules = 6.0 × 1023 x 0.5
= 3.0 × 1023
It is important to be clear about the particles involved. For example, 3.0 × 1023 water molecules contain 9.0 × 1023 atoms. This is because a water molecule, H2O, contains three atoms.
Question
Calculate the number of oxygen atoms in 0.5 moles of oxygen molecules, O2.
Number of atoms = 2 × the number of molecules
Number of molecules = 0.5 × 6.0 × 1023
So number of atoms = 2 × 0.5 × 6.0 × 1023 = 6.0 x 1023
Question
Give the answer in standard form.
3 x 6.0 x 1023 = 18 × 1023
Standard form has a single digit multiplied by the power of ten, so in standard form the number of molecules is 1.8 × 1024.