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S4 Chemistry: Acids, Bases and Salts (Paper 2)
S4 Chemistry: Acids, Bases and Salts (Paper 2)
This paper covers neutralisation, titration calculations, naming salts and choosing an indicator.
Direct answer Balance the equation to get the mole ratio, work out moles from concentration and volume, and choose the indicator by the pH at the end point.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 25 |
| Difficulty mix | Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Acids, Bases and Salts |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat is the reaction called when an acid and a base give a salt and water?
- FoundationWhich salt forms when hydrochloric acid is neutralised by sodium hydroxide?
- FoundationWhat volume of 0.20 M sodium hydroxide neutralises 25.0 cm3 of 0.10 M sulphuric acid?
- FoundationPhenolphthalein is used while sodium hydroxide is added to an acid. How does the colour change at the end point?
- Intermediate20.0 cm3 of sodium hydroxide needs 18.0 cm3 of 0.100 M hydrochloric acid. Find its concentration.
- Intermediate0.050 mol of calcium carbonate reacts with excess hydrochloric acid. How many moles of carbon dioxide form?
- IntermediateWrite the equation for nitric acid with potassium hydroxide and name the salt.
- ChallengeWhat volume of 0.100 M hydrochloric acid reacts completely with 25.0 cm3 of 0.100 M sodium carbonate?
- Challenge0.10 M sodium hydroxide is used to titrate 25.0 cm3 of 0.10 M acetic acid and the end point has a pH above 7. Explain why and choose an indicator.
- Error redoError repair: a student says any neutralisation gives pH 7. Give two counter examples.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | neutralisation | Hydrogen ions from the acid join hydroxide ions from the alkali to form water, and this is neutralisation. |
| 2 | sodium chloride | Chloride from the acid and sodium from the alkali give sodium chloride. |
| 3 | 25.0 cm3 | The acid gives 0.0050 mol of hydrogen ions, so 0.0050 mol of alkali is needed, and dividing by 0.20 gives 0.025 dm3. |
| 4 | from colourless to pink | Phenolphthalein is colourless in acid and turns pink in slight alkali. |
| 5 | 0.0900 M | The acid moles are 0.100 times 0.0180 or 0.00180, and the one to one ratio gives 0.0900 M. |
| 6 | 0.050 mol | In the equation one carbonate gives one carbon dioxide, so the moles are equal. |
| 7 | HNO3 + KOH gives KNO3 + H2O; potassium nitrate | Hydrogen and hydroxide ions form water, leaving nitrate and potassium ions to make potassium nitrate. |
| 8 | 50.0 cm3 | There is 0.0025 mol of carbonate, the ratio is one to two, so 0.0050 mol of acid is needed, that is 0.0500 dm3. |
| 9 | the sodium acetate formed hydrolyses to give an alkaline solution, so phenolphthalein is used | A weak acid salt forms and hydrolyses to give a slightly alkaline solution of pH about 8 to 9, which suits phenolphthalein. |
| 10 | ammonium chloride solution is acidic and sodium acetate solution is alkaline | Only equal amounts of a strong acid and a strong base give about pH 7, because the salt of a weak partner hydrolyses. |
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.
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