Section 1 of 6What is a mole?
Counting unit
- Counting unit A mole is the amount of substance that contains 6.02 × 1023 particles.
- These particles can be atoms, molecules, ions or electrons.
Avogadro constant
- Avogadro constant 6.02 × 1023 per mole is called the Avogadro constant.
- It is chosen so that one mole of a substance has a mass in grams equal to its relative formula mass.
Why chemists use it
- Why chemists use it Atoms are far too small to count individually, but moles can be found by weighing.
Examples
- Examples One mole of carbon is 12 g and contains 6.02 × 1023 atoms.
- One mole of water is 18 g and contains 6.02 × 1023 molecules.
Section 2 of 6Key equations
| Quantity | Equation | Units |
|---|---|---|
| Amount of substance | n = m ÷ Mr | mol |
| Mass | m = n × Mr | g |
| Relative formula mass | Mr = m ÷ n | none (or g/mol as molar mass) |
| Number of particles | N = n × 6.02 × 1023 | number of particles |
Section 3 of 6Calculating moles from mass
1
Find the relative formula mass of the substance from the Ar values.
2
Write the mass in grams. If it is in kilograms, multiply by 1000.
3
Divide mass by Mr to find the number of moles.
4
Example: 11 g of CO2 has Mr = 44, so moles = 11 ÷ 44 = 0.25 mol.
5
To go back, multiply moles by Mr: 0.25 × 44 = 11 g.
Section 4 of 6Number of particles
1Start with mass
- Start with mass For example, 36 g of water.
2Convert to moles
- Convert to moles Mr of water is 18, so moles = 36 ÷ 18 = 2.0 mol.
3Multiply by the constant
- Multiply by the constant Number of molecules = 2.0 × 6.02 × 1023 = 1.20 × 1024.
4Count atoms if needed
- Count atoms if needed Each water molecule has three atoms, so multiply by three to count atoms.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Mole | The unit of amount of substance; one mole contains 6.02 × 1023 particles. |
| Avogadro constant | The number of particles in one mole, 6.02 × 1023. |
| Relative formula mass (Mr) | The sum of the relative atomic masses of the atoms in a formula. |
| Molar mass | The mass of one mole of a substance, in grams per mole. |
| Amount of substance | A quantity measured in moles. |
Section 6 of 6Exam technique
1
Using the wrong formula arrangement, such as mass × Mr to find moles instead of mass ÷ Mr.
2
Forgetting that a mole of a diatomic molecule such as O2 has Mr of 32, not 16.
3
Using mass in kilograms rather than grams.
4
Confusing the number of molecules with the number of atoms.
5
Giving a mole as a mass. A mole is an amount, a number of particles; its mass depends on the substance.