Stoichiometry & the Mole

What this lesson covers

Stoichiometry & the Mole is a 7-part O Level Chemistry lesson. Each part makes one point and then hands over to the next, so it can be read in a sitting or picked up again at the part you stopped on. It defines the idea in plain words before using it, works through an example step by step, breaks the topic into its parts and compares them, names the mistakes that cost marks in an exam, ends with a recap of what to remember.

How it begins

A balanced equation has the same number of each atom on both sides. Balance by changing the numbers in front (coefficients), never the subscripts inside a formula. Mg + O₂ → MgO becomes 2Mg + O₂ → 2MgO, not Mg + O₂ → MgO₂. State symbols (s), (l), (g), (aq) are marks when asked for. Ionic equations keep only the species that change. Acid–alkali neutralisation is always H⁺(aq) + OH⁻(aq) → H₂O(l).

The 7 parts

1. Formulae and equations first: the arithmetic stands on them
2. The mole, three ways in, three ways out
3. Reacting-mass and gas-volume calculations, laid out for marks
4. Empirical and molecular formulae, and titrations
5. Where marks go
6. What to remember
7. Watch one worked through