1.2.5Building Blocks for Understanding
Deducing Stoichiometry (HT Only) — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 7 parts Lesson 11 of 14
Part 1 of 5What is stoichiometry? (HT only)
Meaning
- Meaning Stoichiometry is the ratio in which substances react and form products, shown by the coefficients in a balanced equation.
From data
- From data If experimental masses are known, the mole ratio can be calculated and the equation worked out without being told it.
Why moles
- Why moles Equations compare numbers of particles, so masses have to be converted into moles before they can be compared.
Part 2 of 5Method (HT only)
1
Convert each mass to moles using moles = mass ÷ Mr.
2
Divide all the mole values by the smallest one.
3
Round to the simplest whole-number ratio; if one value ends in .5, double them all.
4
Use these numbers as the coefficients in front of each formula.
5
Check that atoms balance on both sides.
Part 3 of 5Worked example (HT only)
| Substance | Mass (g) | Mr | Moles | Ratio |
|---|---|---|---|---|
| Magnesium | 4.8 | 24 | 0.20 | 2 |
| Oxygen (O2) | 3.2 | 32 | 0.10 | 1 |
| Magnesium oxide | 8.0 | 40 | 0.20 | 2 |
Part 4 of 5Reading the result (HT only)
1Ratio found
- Ratio found 2 : 1 : 2 for Mg : O2 : MgO.
2Equation written
- Equation written 2Mg + O2 → 2MgO.
3Conservation check
- Conservation check Reactants 4.8 + 3.2 = 8.0 g, matching the product mass.
Part 5 of 5Where the data comes from (HT only)
Experimental masses
Heating a metal in a crucible
- Heating a metal in a crucible Weigh the crucible and metal before and after heating;
- the rise in mass is the mass of oxygen that combined.
Reacting masses given in a question
- Reacting masses given in a question Exam questions often supply the mass of each substance, so the first job is always converting every one to moles.
Gas masses
- Gas masses If a gas is produced, its mass may be found by the mass lost from an open flask, then converted into moles in the same way.
Checking reliability
- Checking reliability Repeat the experiment and use a mean, because incomplete reactions or escaping product can distort the ratio.
Key terms
| Term | Meaning |
|---|---|
| Stoichiometry | The quantitative relationship between amounts of reactants and products. |
| Mole ratio | The ratio of moles of substances in a balanced equation. |
| Coefficient | The number in front of a formula in an equation. |
| Simplest ratio | The smallest whole numbers in the same proportion. |
Avoid these mistakes
1
Using the ratio of masses instead of the ratio of moles.
2
Not dividing by the smallest number, leaving awkward decimals.
3
Rounding 1.5 down to 1 rather than doubling every value to remove the half.
4
Forgetting that oxygen gas is O2 when finding its Mr.
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