The Mole and Avogadro's Constant
Simple Explanation
A mole is a counting unit for atoms and molecules, just like 'a dozen' means 12. One mole of anything contains 6.02 × 10²³ particles — this number is called Avogadro's constant.
Why Do We Need It?
Atoms and molecules are far too small and numerous to count individually, so chemists use the mole to work with practical, everyday amounts of substance instead.
Worked Example
Find the number of molecules in a sample
How many molecules are there in 2 moles of water?
Why Does This Work?
Since a mole is defined as a fixed number of particles, multiplying that fixed number by however many moles you have always gives the total particle count — the same logic as multiplying 12 by the number of dozens.
Real-Life Example
Comparing a mole of different substances
A mole of water (18 g) is a small cup; a mole of carbon (12 g) is a small lump.
Even though these masses look very different, both contain exactly the same number of particles — 6.02 × 10²³ — because the mole always counts particles, not mass.
Practice
How many atoms are in 3 moles of carbon? Give your answer as a number of ×10²³ (e.g. enter 12.0 for 12.0 × 10²³).
MediumCommon mistake
Thinking a mole is a fixed mass — it's actually a fixed NUMBER of particles; the mass of one mole is different for every substance.
Quick Review
- A mole is a counting unit: 1 mole = 6.02 × 10²³ particles (Avogadro's constant).
- The mole lets chemists work with countable, practical amounts.
- 1 mole of any substance always has the same number of particles, but a different mass.