Mole: definition in chemistry
Mole: The SI unit for amount of substance. One mole contains exactly 6.02214076 × 10²³ particles, often rounded to 6.022 × 10²³ (Avogadro's number).
Atoms and molecules are far too small to count one by one, so chemists count them in moles, much as eggs are counted in dozens. The mass of one mole of a substance in grams, its molar mass, is numerically equal to its formula mass in atomic mass units. This lets you convert between the grams you can weigh and the numbers of particles that react.
Examples
- 1 mole of carbon-12 has a mass of 12 grams
- 1 mole of water (H₂O) has a mass of about 18 grams
- 1 mole of an ideal gas fills about 22.4 liters at 0 °C and 1 atm
- In 2H₂ + O₂ → 2H₂O, 2 moles of hydrogen react with 1 mole of oxygen
On the periodic table
The atomic mass of each element on the table is also its molar mass in grams per mole, for example about 16.00 g/mol for oxygen.