The Mole Concept

Key definitions

• One mole of a substance contains $6.02 \times 10^{23}$ particles (the Avogadro constant).
• Molar mass ($M$) is the mass of one mole, in g/mol.

Core equation

Worked example

How many moles are in 9.0 g of water?

$M(\ce{H2O}) = 2(1.0) + 16.0 = 18.0$ g/mol, so $n = 9.0 / 18.0 = 0.50$ mol.

Reacting masses

The equation tells us 2 mol of hydrogen react with 1 mol of oxygen.