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S5 Chemistry: Ionic Precipitation (Paper 4: Misconception Drill)
S5 Chemistry: Ionic Precipitation (Paper 4: Misconception Drill)
Ten items on the commonest precipitation misconceptions: solubility in acid, carbonate solubility, the limiting reagent, remaining moles and whether heating is needed, with answers.
Direct answers Answers in order: insoluble, insoluble, no since only sodium potassium and ammonium carbonates dissolve, 0.287 g, 20 cm3, reddish-brown, 0.583 g, 0.0005 mol, hydration energy overcomes lattice energy, mixing alone suffices.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S5 Chemistry: Ionic Precipitation |
| Level | S5 |
02 Questions (10 items)
- FoundationDoes silver chloride dissolve in dilute nitric acid?
- FoundationDoes barium sulphate dissolve in dilute hydrochloric acid?
- FoundationIs it true that all carbonates are soluble?
- IntermediateWhat mass of silver chloride, Mr = 143.5, forms from 10.0 cm3 of 0.20 M silver nitrate with excess sodium chloride?
- IntermediateWhat volume of 0.50 M barium chloride is needed to precipitate 0.010 mol of sulphate ions?
- IntermediateWhat colour is the precipitate when iron(III) ions are treated with sodium hydroxide solution?
- ChallengeWhen 20.0 cm3 of 0.15 M barium chloride is mixed with 25.0 cm3 of 0.10 M sodium sulphate, what mass of barium sulphate, Mr = 233, forms?
- ChallengeFrom the previous question, how many moles of barium ions are left after the reaction?
- ChallengeWhy are sodium, potassium and ammonium salts generally soluble in water?
- Error redoA student thinks heating is necessary to form a precipitate. How should this be corrected?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | No, it does not. | That is why a white precipitate that survives dilute nitric acid is confirmed as silver chloride. |
| 2 | No, it does not. | Barium sulphate is extremely insoluble and stays as a white precipitate in acid, which is the basis of the sulphate test. |
| 3 | No. Only sodium, potassium and ammonium carbonates are soluble; most others are not. | Calcium carbonate and copper carbonate are insoluble, which is why carbonates are used to precipitate metal ions. |
| 4 | 0.287 g | There is 0.0020 mol of silver ions, giving the same moles of silver chloride, which is 0.287 g. |
| 5 | 20 cm3 | Sulphate and barium react one to one, so 0.010 mol of barium is needed; dividing by 0.50 M gives 0.020 dm3. |
| 6 | A reddish-brown precipitate of iron(III) hydroxide. | Metal hydroxides differ in colour: copper blue, iron(II) green, iron(III) reddish-brown, so they help identify ions. |
| 7 | 0.583 g | There are 0.0030 mol of barium ions but only 0.0025 mol of sulphate, so 0.0025 mol times 233 gives 0.5825 g. |
| 8 | 0.0005 mol | 0.0030 mol minus the 0.0025 mol that reacted leaves 0.0005 mol in solution. |
| 9 | Because the hydration energy of these ions is enough to overcome the lattice energy, so the crystal breaks up in water. | Solubility depends on lattice energy against hydration energy, and for these ions hydration wins. |
| 10 | Mixing the two solutions is enough; heating only changes the rate and the particle size. | Precipitation is governed by solubility rather than temperature, and heating can even dissolve some precipitates. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Turn every wrong item today into one sentence: I thought this, but actually that.Tutor Registration →
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