Unit 3
The mole.
Counting atoms too small to see: formula mass, the mole and the Avogadro constant, and empirical formulas from measured masses.
Before this unit
- Atoms and elements
- Ratio and scale Mathematics
The unit, lesson by lesson
Lessons
4 lessons · 21 practice questions · about 2 hours
Relative masses
An atom weighs about 10⁻²³ grams, so chemists compare masses instead of weighing atoms: relative atomic mass, and a compound’s relative formula mass, added up from its formula.
Counting by weighing
The mole: a fixed number of particles, 6.02 × 10²³, chosen so that a mole of a substance weighs its formula mass in grams. How to weigh out a mole, and why a mole of gold is heavier than a mole of aluminium.
Grams, moles and particles
Two formulas link what you weigh to what you count: n = m ÷ M and N = n × Nₐ. Working both ways, molecules against atoms, and checking with units.
Formulas from masses
How much of a compound’s mass belongs to each element: percentage composition and an element’s mass in a sample, and how measured masses become an empirical formula and a molecular formula.