Mole Concept — Part 2: The mole, molar volume and vapour density: visual lesson, ICSE Class 10 Chemistry (Science Paper 2)

Lesson

Counting by weighing

Part 2 — atoms and molecules are far too small to count one by one, so chemists count them in huge bundles called moles, by weighing.

In this lesson you will

Define the mole and Avogadro's number
Distinguish atomic mass from gram atomic mass, and molecular mass from gram molecular mass
Convert between mass, moles, number of particles and volume of a gas at STP
Use vapour density to find relative molecular mass

Concept

Mole: the amount of a substance that contains as many particles (atoms, molecules or ions) as there are atoms in exactly 12 g of carbon-12.

This number is Avogadro's number, $N_\text{A} = 6.022 \times 10^{23}$ (often rounded to $6 \times 10^{23}$ ).

Concept

Atomic mass (relative atomic mass): the mass of one atom compared with one-twelfth of the mass of a carbon-12 atom; sodium = 23 (u).
Gram atomic mass: the atomic mass expressed in grams, the mass of 1 mol of atoms; sodium = 23 g.
Molecular mass (relative molecular mass): the sum of the atomic masses in one molecule; water = 2 × 1 + 16 = 18 (u).
Gram molecular mass (molar mass): the molecular mass in grams, the mass of 1 mol of molecules; water = 18 g.

Table comparing atomic or molecular mass with gram atomic or gram molecular mass for sodium, water and carbon dioxide
Substance	Atomic or molecular mass	Gram atomic or molecular mass	Particles in that mass
Sodium, Na	23 u	23 g	6.022 × 10²³ atoms
Water, H₂O	18 u	18 g	6.022 × 10²³ molecules
Carbon dioxide, CO₂	44 u	44 g	6.022 × 10²³ molecules