A mole (mol) is the unit that tells us how many particles we have. Think of it as a “big bag” that holds a specific number of items. Just like a bag of 12 marbles, a mole holds a fixed number of atoms, ions, or molecules.
The number of particles in one mole is called the Avogadro constant and is written as:
\$N_A = 6.02 \times 10^{23}\ \text{particles mol}^{-1}\$
That means if you have one mole of water molecules, you have \$6.02 \times 10^{23}\$ water molecules – a number so huge it’s hard to imagine!
Suppose you have 18.02 g of water (H₂O). The molar mass of water is 18.02 g mol⁻¹, so:
\$n = \dfrac{18.02\ \text{g}}{18.02\ \text{g mol}^{-1}} = 1\ \text{mol}\$
That 1 mol of water contains \$6.02 \times 10^{23}\$ molecules – the same number as the Avogadro constant.
Tip: When converting grams to moles, always divide by the molar mass. When converting moles to particles, multiply by \$6.02 \times 10^{23}\$.
Remember: Mass (g) ÷ Molar Mass (g mol⁻¹) = Moles (mol)
Moles (mol) × Avogadro Constant (particles mol⁻¹) = Particles (atoms/molecules)
| Compound | Formula | Molar Mass (g mol⁻¹) |
|---|---|---|
| Water | H₂O | 18.02 |
| Carbon Dioxide | CO₂ | 44.01 |
| Sodium Chloride | NaCl | 58.44 |
Answer these before the next lesson to test your understanding! 🚀