1.2.4Building Blocks for Understanding
Amounts In Moles (HT Only) — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 8 parts Lesson 10 of 14
Section 1 of 8The mole (HT only)
Definition
- Definition One mole is the amount of substance containing 6.02 × 1023 particles, the Avogadro constant.
- The particles may be atoms, molecules, ions or electrons.
Link to mass
- Link to mass One mole of any substance has a mass in grams equal to its Ar or Mr.
- So one mole of carbon is 12 g and one mole of water is 18 g.
Why use it
- Why use it Atoms are far too small to count or weigh individually, so the mole lets chemists weigh out masses that contain known numbers of particles.
- Reacting amounts can then be compared fairly.
Section 2 of 8Key equations (HT only)
| Quantity | Equation | Units |
|---|---|---|
| Amount of substance | moles = mass ÷ Mr | mol |
| Mass | mass = moles × Mr | g |
| Number of particles | particles = moles × 6.02 × 1023 | none |
| Relative mass (Ar or Mr) | Mr = mass ÷ moles | g/mol |
Section 3 of 8Calculating moles from mass (HT only)
1
Work out the Mr (or Ar) from the periodic table.
2
Write the mass in grams, converting from kg if required.
3
Divide mass by Mr to get moles.
4
Example: 11 g of CO2 (Mr 44) is 11 ÷ 44 = 0.25 mol.
5
Multiply by 6.02 × 1023 to find the number of molecules, here 1.5 × 1023.
Section 4 of 8Worked examples (HT only)
| Question | Working | Answer |
|---|---|---|
| Moles in 36 g of water (Mr 18) | 36 ÷ 18 | 2.0 mol |
| Mass of 0.50 mol of NaCl (Mr 58.5) | 0.50 × 58.5 | 29.25 g |
| Atoms in 0.10 mol of carbon | 0.10 × 6.02 × 1023 | 6.02 × 1022 |
| Moles in 2.4 kg of Mg (Ar 24) | 2400 ÷ 24 | 100 mol |
Section 5 of 8Moles and equations (HT only)
2Mg + O2 → 2MgO
Ratio of moles
- Ratio of moles The big numbers give the ratio of moles: 2 mol Mg react with 1 mol O2 to give 2 mol MgO.
Scaling
- Scaling Doubling the moles of each substance still keeps the same ratio, so 4 mol Mg need 2 mol O2.
Masses are not in the same ratio
- Masses are not in the same ratio Because each substance has a different Mr, the masses reacting do not follow the coefficients, only the moles do.
Used later
- Used later This allows calculation of reacting masses and identifying limiting reactants.
Section 6 of 8Large and small quantities (HT only)
Using standard form
- Using standard form Numbers of particles are huge,
- so they are written as a number between 1 and 10 multiplied by a power of ten, for example 3.01 × 1023.
- On a calculator use the EXP or ×10x button rather than typing the multiplication out.
Small samples
- Small samples A tiny mass such as 0.0018 g of water is only 1.0 × 10-4 mol, which still contains about 6 × 1019 molecules.
- This shows why individual particles are never counted in practice.
Sense check
- Sense check A mass larger than the Mr in grams gives more than one mole, and a smaller mass gives less than one mole, so check your answer agrees.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Mole | The unit for amount of substance, equal to 6.02 × 1023 particles. |
| Avogadro constant | The number of particles in one mole, 6.02 × 1023 per mole. |
| Molar mass | The mass in grams of one mole of a substance. |
| Amount of substance | A measure, in moles, of the number of particles present. |
| Standard form | A way of writing very large or small numbers as a number between 1 and 10 times a power of ten. |
Section 8 of 8Exam technique
1
Using the mass of a single atom instead of Ar when converting to moles.
2
Forgetting to convert kilograms to grams before dividing.
3
Using the Ar of one atom of oxygen (16) for O2 gas, whose Mr is 32.
4
Writing standard form numbers wrongly, such as 6.02 × 10-23.
5
Giving a number of particles as moles, or moles as a mass, without checking the units.
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