The mole and the Avogadro constant
The mole
- The mole (mol) is the unit for amount of substance.
- One mole contains $6.02 \times 10^{23}$ particles — the Avogadro constant.
- The molar mass is the mass of one mole, in g/mol; its number equals the $A_r$ or $M_r$.
$$\text{amount (mol)} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$$
Practice
One mole of any substance contains:
A mole always contains 6.02 × 10²³ particles — the Avogadro constant.
Worked example
- How many moles are in 36 g of water? (molar mass of water = 18 g/mol)
$$n = \frac{36}{18} = 2 \text{ mol}$$
- To find the number of particles, multiply the moles by the Avogadro constant.
Practice
How many moles are in 88 g of carbon dioxide? (molar mass = 44 g/mol)
amount = mass ÷ molar mass = 88 ÷ 44 = 2 mol.
Practice
What is the mass of 3 mol of water? (molar mass = 18 g/mol)
mass = amount × molar mass = 3 × 18 = 54 g.
You've got it
Key idea
- a mole = $6.02 \times 10^{23}$ particles (the Avogadro constant)
- amount (mol) = mass ÷ molar mass
- 36 g of water ÷ 18 g/mol = 2 mol