The MoleOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is a mole?
A mole is the unit chemists use to count particles (atoms, molecules or ions) in a substance, because particles are far too small and numerous to count individually.
One mole of a substance is defined as its relative formula mass in grams.
For example, the Mr of water is 18, so one mole of water has a mass of 18 g.
Section 2
What is Avogadro's constant?
One mole of any substance contains the same number of particles: 6.02 × 10²³ atoms, molecules or ions.
This number is called the Avogadro constant. It means one mole of carbon atoms contains the same number of particles as one mole of water molecules, even though their masses are very different.
Section 3
How do you calculate the number of moles in a given mass?
Use the equation:
n = m ÷ Mr
where n is the number of moles, m is the mass in grams, and Mr is the relative formula mass.
How many moles are in 9 g of water (Mr = 18)? n = 9 ÷ 18 = 0.5 mol.
Section 4
How do you calculate the mass from a given number of moles?
Rearranging n = m ÷ Mr gives:
m = n × Mr
This is used constantly in reacting mass calculations, once the number of moles of a substance has been worked out from a balanced equation.
What is the mass of 2 mol of carbon dioxide, CO2 (Mr = 44)? m = 2 × 44 = 88 g.
Always write down which form of the equation you are using (n = m/Mr or m = n × Mr) — examiners give credit for correct method even if arithmetic slips occur.
Must Know
- One mole of a substance is its relative formula mass in grams
- One mole of any substance contains 6.02 × 10²³ particles (Avogadro's constant)
- n = m ÷ Mr (moles = mass ÷ relative formula mass)
- m = n × Mr (mass = moles × relative formula mass)
- The mole allows chemists to count and compare particles indirectly by measuring mass
That's the notes covered.
Carry on to the next subtopic.