Mole concept, Avogadro number, molar mass and molar volume
A mole is the chemist's counting unit: one mole holds 6.023 × 10²³ particles, weighs its molar mass in grams and, for a gas at STP, fills 22.4 litres.
A counting unit, like a dozen
A fruit seller counts bananas in dozens (12). Atoms are far too small to count one by one, so chemists count them in moles. One mole is 6.023 × 10²³ particles. This number is called the Avogadro number (N_A). The particles can be atoms, molecules or ions, so always say which one you mean.
One mole of different substances
| One mole of | Number of particles | Molar mass | Volume at STP |
|---|---|---|---|
| Carbon atoms (C) | 6.023 × 10²³ atoms | 12 g | Not a gas |
| Water molecules (H₂O) | 6.023 × 10²³ molecules | 18 g | Not a gas at STP |
| Oxygen gas (O₂) | 6.023 × 10²³ molecules (2 × 6.023 × 10²³ atoms) | 32 g | 22.4 L |
| Carbon dioxide (CO₂) | 6.023 × 10²³ molecules | 44 g | 22.4 L |
| Sodium ions (Na⁺) | 6.023 × 10²³ ions | 23 g | Not a gas |
Molar mass and molar volume
- Molar mass is the mass of one mole of a substance in grams. It equals the atomic or molecular mass written in grams.
- For an element it is the gram atomic mass (O = 16 g). For a compound it is the gram molecular mass (H₂O = 18 g).
- Its unit is g mol⁻¹ (grams per mole).
- Molar volume: one mole of any gas at STP takes up 22.4 L (22400 cm³ or 22.4 dm³).
- STP means standard temperature and pressure: 273 K (0 °C) and 1 atm (760 mm of Hg).
The mole bridge
For a gas at STP: volume = number of moles × 22.4 L. Every conversion passes through moles.
A glass of water (about 180 mL, which weighs 180 g) is 10 moles of water. That is about 6 × 10²⁴ molecules in one glass.
1 marks: “The volume occupied by 1 mole of any gas at STP is ______.” 22.4 litres.
2 marks: “Define mole.” One mole is the amount of substance that contains as many particles (6.023 × 10²³) as there are atoms in exactly 12 g of carbon-12.
4 marks: “Define molar mass and molar volume. How many molecules are present in 36 g of water?” Two definitions. Moles = 36 ÷ 18 = 2. Molecules = 2 × 6.023 × 10²³ = 1.2046 × 10²⁴.
One mole: 6.023 × 10²³ particles, its molar mass in grams, and 22.4 L if it is a gas at STP.