Matter and Reactions
Moles & Balanced Equations
14 min · 2 worked examplesFree
The counting unit chemistry runs on, and how to work out how much of one substance reacts with how much of another.
After this lesson you can
- Convert between grams and moles
- Balance an equation and say why it must balance
- Work out which reagent limits the yield
The mole
Atoms are far too small to count, so chemistry counts them in bulk. A mole is 6.02 × 10²³ particles. Avogadro's number.
The reason for that particular figure is convenience: one mole of an element has a mass in grams equal to its relative atomic mass. Carbon is 12, so one mole of carbon is 12 g. Water is 18, so one mole is 18 g.
So moles = mass ÷ molar mass, and that one relationship converts between what you can weigh and what actually reacts. Reactions happen in whole-number ratios of particles, not of grams, which is why every quantitative chemistry calculation passes through moles.
In the full lesson
- The mole
- Balancing equations
- Limiting reagent and yield
Free with an account. Read it, work the examples, answer the questions, then build it in the simulator.