1.5.4Principles of Chemistry
Calculate Reacting Masses — Edexcel International GCSE Science Double Award 4SD0 - Chemistry Revision Notes & Topical Questions
5 min read 11 parts Lesson 25 of 50
Part 1 of 5The three-step approach
1Moles of the known substance
- Moles of the known substance Convert the given mass into moles using moles = mass ÷ Mr.
2Use the equation ratio
- Use the equation ratio The big numbers in the balanced equation give the mole ratio between the known and the wanted substance.
3Convert to mass
- Convert to mass Multiply the moles of the wanted substance by its Mr to give the mass in grams.
Part 2 of 5Worked example: burning magnesium
1
Question: what mass of magnesium oxide forms when 6.0 g of magnesium burns completely? 2Mg + O2 → 2MgO (Ar: Mg = 24, O = 16).
2
Moles of Mg = 6.0 ÷ 24 = 0.25 mol.
3
The equation shows 2 mol Mg gives 2 mol MgO, a 1 : 1 ratio, so 0.25 mol MgO forms.
4
Mr of MgO = 24 + 16 = 40, so mass = 0.25 × 40 = 10 g.
Part 3 of 5Worked example: a ratio that is not 1:1
| Step | Working |
|---|---|
| Equation | 2Al2O3 → 4Al + 3O2 |
| Question | Maximum mass of aluminium from 51 tonnes of Al2O3 (Ar: Al = 27, O = 16) |
| Convert mass | 51 tonnes = 51 000 000 g |
| Mr of Al2O3 | (2 × 27) + (3 × 16) = 102 |
| Moles of Al2O3 | 51 000 000 ÷ 102 = 500 000 mol |
| Ratio | 2 mol Al2O3 gives 4 mol Al, so moles Al = 2 × 500 000 = 1 000 000 |
| Mass of Al | 1 000 000 × 27 = 27 000 000 g = 27 tonnes |
Part 4 of 5Finding the equation from masses
Masses → moles → simplest ratio
Convert each mass
- Convert each mass Divide the mass of each reactant and product by its Mr to find moles.
Divide by the smallest
- Divide by the smallest Divide all mole values by the smallest one to find the simplest whole-number ratio.
Write the equation
- Write the equation Use the ratio as the big numbers in front of each formula, then check that the atoms balance.
Part 5 of 5Exam wording and good habits
Key phrases
- Key phrases 'Calculate the mass', 'minimum mass' and 'maximum mass' all signal this same moles, ratio, mass method.
Units
- Units Convert tonnes to grams (1 tonne = 106 g) or kilograms to grams before starting, and convert back at the end if asked.
Check the equation
- Check the equation Make sure the equation is balanced before using the ratio, otherwise the answer will be wrong.
Key terms
| Term | Meaning |
|---|---|
| Mole ratio | The ratio of the amounts in moles of substances in a balanced equation. |
| Reacting mass | The mass of a reactant or product involved in a reaction. |
| Balanced equation | An equation with equal numbers of each type of atom on both sides. |
| Relative formula mass | The sum of the relative atomic masses in a formula. |
| Tonne | A unit of mass equal to 1 000 000 g. |
Avoid these mistakes
1
Using the mass ratio of the equation instead of the mole ratio. Always convert to moles first.
2
Using the wrong Mr at the last step, such as the reactant's Mr instead of the product's.
3
Forgetting to multiply the Mr by the coefficient. The ratio comes from the big numbers only.
4
Not converting tonnes or kilograms to grams, which is a power-of-ten error.
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