The Mole: Chemistry's Dozen
Atoms are too small to count, so chemists count in batches of 6 × 10²³ — a batch sized so the scale does the counting for you.
Builds on: 0.1 Atoms: The Alphabet of Matter2.1 Chemical Reactions & Balancing
A counting unit, nothing more
A pair is 2, a dozen is 12, a mole is 6.022 × 10²³ — that’s the whole definition. The number (Avogadro’s number, NA) looks absurd, but it’s chosen with surgical cunning: one mole of any substance weighs its atomic/molecular mass in grams. Carbon-12 atoms have mass number 12 → one mole of carbon weighs 12 g. Water molecules weigh 18 u → one mole of water is 18 g, about one gulp.
This is the bridge between the invisible and the weighable. You cannot count molecules, but you can weigh 18 g of water — and then you know, with certainty, that you hold 6.022 × 10²³ molecules. The scale becomes a particle counter.
Molar mass from a formula
Add up the atomic masses from the periodic table. Water H₂O: 2 × 1.008 + 16.00 = 18.02 g/mol. Carbon dioxide CO₂: 12.01 + 2 × 16.00 = 44.01 g/mol. Table sugar C₁₂H₂₂O₁₁: 342.3 g/mol. That’s the entire skill — addition with a map.
Counting particles
A mole of marshmallows would cover Germany roughly 1000 km deep. A mole of water molecules fits in a shot glass. That contrast — a number too big to imagine, hiding inside amounts you handle daily — is why chemistry needs the mole: reality operates in armies, and the mole is the army’s name.
⚗️ Lab — The Particle Counter
A scale on the left, the conversion chain on the right.
- Set water to exactly 18 g — read off precisely one mole.
- Same 100 g of water vs. iron: which holds more particles, and why?
- Watch the particle count: it never leaves the 10²³–10²⁴ neighbourhood for lab-sized amounts.
Work it by hand — 0 / 4
No multiple choice here: compute the value and type it. Suffixes like 2.5k, 20m or 100µ are understood; answers within ±2% count.