The Mole

Use the mole to count particles and connect the mass of a substance to chemical equations.

About 3 min read
  • Define the mole and the Avogadro constant
  • Calculate moles from mass and molar mass
  • Use moles to find reacting masses
Before you start

You can find relative formula mass from a periodic table and balance chemical equations.

Counting particles

Atoms are too small to count one by one, so chemists count them in moles. One mole of any substance contains 6.02 × 10²³ particles. This number is called the Avogadro constant. A mole works like the word dozen, but it represents a far larger number.

Definition: mole

The amount of substance containing 6.02 × 10²³ particles.

Definition: molar mass

The mass of one mole of a substance, measured in g/mol.

Moles and mass

The molar mass of a substance has the same value as its relative formula mass. Carbon dioxide has Mr = 44, so one mole has a mass of 44 g.

Moles from mass

moles = mass ÷ molar mass

Reacting masses

What mass of magnesium oxide forms from 4.8 g of magnesium?

Maths
2Mg + O₂ → 2MgO
moles of Mg = 4.8 ÷ 24 = 0.20 mol
mole ratio Mg : MgO = 2 : 2
moles of MgO = 0.20 mol
mass of MgO = 0.20 × 40 = 8.0 g

Answer: 8.0 g

Use the equation ratio

Convert to moles before using the ratio. Masses do not follow the balancing numbers directly.

Exam focus
Say it like this“Moles of CO₂ = 11 ÷ 44 = 0.25 mol”
Not like this“11 × 44 = 484 mol” — Divide mass by molar mass.
Common mistakeUsing the mass ratio from the equation before converting masses into moles.

Practice

How many moles are in 36 g of water? Mr = 18.
36 ÷ 18 = 2.0 mol.
Find the mass of 0.50 mol of NaCl. Mr = 58.5.
0.50 × 58.5 = 29.25 g.
How many atoms are in 2.0 mol of helium?
2.0 × 6.02 × 10²³ = 1.204 × 10²⁴ atoms.
Going further
Calcium carbonate decomposes: CaCO₃ → CaO + CO₂. Find the mass of CaO from 25 g of CaCO₃.
Moles of CaCO₃ = 25 ÷ 100 = 0.25 mol. The ratio is 1:1, so mass of CaO = 0.25 × 56 = 14 g.

Quick check

The mole quick check

What is the Avogadro constant?

This is the number of particles counted in one mole of a substance.

How many moles are in 12 g of magnesium? Ar = 24.

Divide the mass by molar mass, so 12 is divided by 24.

What is the molar mass of CO₂?

Add one carbon at 12 and two oxygens at 16 each.
Key takeaways
  • One mole contains 6.02 × 10²³ particles.
  • Moles = mass ÷ molar mass.
  • Equation ratios compare moles, not masses.
  • Convert back to mass at the end.
Back to Grade 11 chaptersRelated resources →