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S4 Chemistry: Acids, Bases and Salts (Paper 3)
S4 Chemistry: Acids, Bases and Salts (Paper 3)
This paper compares strong and weak acids and bases, looks at conductivity, safe dilution and everyday neutralisation.
Direct answer Compare ionisation rather than concentration, and for titration sums get the hydrogen ion concentration from the pH first.
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)
- FoundationWhich conducts electricity better at the same concentration, hydrochloric acid or acetic acid?
- FoundationWhat is the main acid in stomach juice?
- FoundationWhich household substance neutralises the acid of an ant sting?
- FoundationWhen diluting concentrated sulphuric acid, should the acid go into water or water into the acid?
- IntermediateWhich has the lower pH, 0.10 M hydrochloric acid or 0.10 M acetic acid?
- Intermediate0.10 M ammonia has a pH of about 11.1 while 0.10 M sodium hydroxide has 13. Explain the difference.
- IntermediateWhy is limestone used to neutralise acidic soil?
- ChallengeWhat volume of 0.10 M sodium hydroxide neutralises 100 cm3 of a strong acid of pH 3?
- ChallengeAt 0.10 M, which has the lower pH and the better conductivity, sulphuric acid or hydrochloric acid? Why?
- Error redoError repair: a student says any solution of pH 7 is neutral. When is that true?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | hydrochloric acid | Hydrochloric acid is a strong electrolyte and ionises completely, so it gives more ions and conducts better. |
| 2 | hydrochloric acid | The stomach lining secretes hydrochloric acid at about pH 1.5 to 3.5 for killing microbes and digesting food. |
| 3 | dilute baking soda solution | Ant stings are acidic, so a weak base such as sodium hydrogencarbonate neutralises them gently. |
| 4 | add the acid slowly to the water | Dilution releases much heat, so adding water to concentrated acid can make it boil and spit, which is dangerous. |
| 5 | hydrochloric acid | The strong acid gives 0.10 M hydrogen ions while the weak acid gives only about 1.3 times ten to the minus three molar. |
| 6 | ammonia is a weak base and is only partly ionised | Ammonia has a small Kb of 1.8 times ten to the minus five, so its hydroxide concentration and pH are much lower. |
| 7 | the carbonate reacts with the acid and removes hydrogen ions | Calcium carbonate reacts with soil acid to give a calcium salt, water and carbon dioxide, raising the pH. |
| 8 | 1.0 cm3 | There are 10 to the minus four moles of hydrogen ions, so 0.0010 dm3 or 1.0 cm3 of alkali is needed. |
| 9 | sulphuric acid, because it supplies two protons and gives a lower pH of about 0.89 | Sulphuric acid is diprotic, so even with partial second ionisation the total hydrogen ion concentration is about 0.13 M against 0.10 M for the hydrochloric acid. |
| 10 | only at 25 degrees Celsius, because the neutral pH falls as Kw rises | Kw rises with temperature, so at 50 degrees Celsius the neutral pH is about 6.63 and a pH of 7 is slightly alkaline. |
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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