Quick Summary
What you need to know: This chapter is about measuring chemical amounts using the mole, and how to convert between mass, particles, volume, and concentration. Everything here connects back to one core idea: the mole lets us count atoms and molecules by weighing them instead.
The Mole
A unit of amount of substance. 1 mole = 6.02 × 10²³ particles. This number is the Avogadro Constant.
Molar Mass
The mass of one mole of a substance. For elements: equals the relative atomic mass in grams. For compounds: sum of all atoms in grams.
Moles ↔ Mass
Moles = Mass ÷ Mr — This is the bridge between what you weigh and what you count.
Gas Volume
At RTP, 1 mole of any gas = 24 dm³. This applies to gases only, and only at room conditions.
Concentration
How much solute in a solution. Can be in g/dm³ or mol/dm³. Key for titrations and reactions in solution.
Reacting Masses & Yields
Use balanced equations and mole ratios to calculate how much product forms, and what percentage you actually get.
The Mole & Avogadro's Constant
What is the Mole?
The mole (symbol: mol) is the SI unit of amount of substance. It's how chemists count particles — atoms, molecules, or ions.
Think of it like a baker's dozen: a baker doesn't sell individual buns; they sell by the dozen (12 buns). A chemist doesn't count individual atoms; they count by the mole. And just as a dozen always means 12, a mole always means the same number of particles: 6.02 × 10²³.
Avogadro's Constant
One mole contains 6.02 × 10²³ particles. This number is called the Avogadro Constant (symbol: NA). It never changes.
- 1 mole of sodium atoms = 6.02 × 10²³ Na atoms
- 1 mole of oxygen molecules = 6.02 × 10²³ O₂ molecules
- 1 mole of sodium chloride ions = 6.02 × 10²³ NaCl formula units
Why is this number useful? Because 6.02 × 10²³ is exactly the number of atoms in one gram of carbon-12. This makes the mole the perfect bridge between the atomic world (where we count particles) and the laboratory world (where we weigh things).
Molar Mass
The molar mass is the mass of one mole of a substance. It's measured in g/mol, and it has a simple rule:
Examples:
- Carbon: Ar = 12, so 1 mole of C atoms = 12 g
- Helium: Ar = 4, so 1 mole of He atoms = 4 g
- Water (H₂O): Mr = (2 × 1) + 16 = 18, so 1 mole of H₂O = 18 g
- Carbon dioxide (CO₂): Mr = 12 + (2 × 16) = 44, so 1 mole of CO₂ = 44 g
Memory trick: The molar mass in g/mol is always the same number as Mr. Just put "g/mol" on the end. Simple as that.
Linking Moles, Mass & Mr
The Core Equation
This is the most important equation in this chapter. It lets you convert between mass (what you weigh) and moles (what you count):
Or rearranged:
- Mass = Moles × Mr
- Mr = Mass ÷ Moles