中文（繁體）
中文（简体）
English
S4 Chemistry Acids and Alkalis, Paper 3: mixtures, dilution and purity
S4 Chemistry Acids and Alkalis, Paper 3: mixtures, dilution and purity
Ten graded items on mixed solutions, successive dilution and sample purity, all built on mole arithmetic.
Focus For mixtures find both amounts in moles and subtract; dilution changes concentration but not the amount; purity works back from the titration volume.
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, Alkalis and Neutralisation |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationFind the pH of a 0.010 mol dm^-3 hydrochloric acid solution.
- FoundationFind the pH at 25 degrees of a 0.10 mol dm^-3 sodium hydroxide solution.
- FoundationWhat colour does universal indicator show in a solution of pH 12?
- Foundation25.0 cm^3 of 0.100 mol dm^-3 sulphuric acid is neutralised by 0.100 mol dm^-3 sodium hydroxide. What volume is needed?
- Intermediate25.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid is mixed with 25.0 cm^3 of 0.200 mol dm^-3 sodium hydroxide. Find the pH.
- IntermediateFind the approximate pH of 0.10 mol dm^-3 ethanoic acid with Ka equal to 1.8 x 10^-5.
- Intermediate10.0 cm^3 of acid at pH 2 is diluted to 1 000 cm^3 with water. Find the pH.
- ChallengeA 1.00 g sample of impure calcium carbonate, molar mass 100 g mol^-1, reacts with 180 cm^3 of 0.100 mol dm^-3 hydrochloric acid. Find the percentage purity.
- Challenge100 cm^3 of 0.050 mol dm^-3 hydrochloric acid is mixed with 100 cm^3 of 0.050 mol dm^-3 sulphuric acid. Find the pH, taking volumes as additive.
- Error redoA student says diluting a pH 3 acid a thousandfold gives pH 6, so more dilution must turn it alkaline. Give the correct answer.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | pH 2.0 | The acid ionises fully, so the hydrogen ion concentration is 0.010 and its negative logarithm is 2.0. |
| 2 | pH 13.0 | The hydroxide concentration is 0.10 so the pOH is 1.0, and pH plus pOH equals 14 at 25 degrees. |
| 3 | Purple (deep blue-purple) | Universal indicator turns purple in strong alkali, green near neutral and red in acid. |
| 4 | 50.0 cm^3 | The acid is 0.0025 mol and gives 0.0050 mol of hydrogen ions, so 50.0 cm^3 of alkali is needed. |
| 5 | pH 12.70 | The alkali is 0.0050 mol against 0.0025 mol of acid, leaving 0.0025 mol of hydroxide in 0.050 dm^3, so the pOH is 1.30. |
| 6 | About pH 2.9 | The weak acid ionises only partly, so the hydrogen ion concentration is about 1.34 x 10^-3 and the pH about 2.87. |
| 7 | pH 4 | The volume grows a hundredfold, so the hydrogen ion concentration falls a hundredfold and the pH rises by two. |
| 8 | 90.0% | The acid is 0.0180 mol, so the carbonate is 0.0090 mol or 0.900 g, which is 90.0 per cent of the sample. |
| 9 | pH 1.12 | The two acids give 0.0150 mol of hydrogen ions in 0.200 dm^3, a concentration of 0.075 and a pH of 1.12. |
| 10 | The pH becomes 6 and the solution is still acidic; further dilution only approaches pH 7 | Diluting a thousandfold gives pH 6, which is still acidic; further dilution with pure water approaches pH 7 but never exceeds it. |
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.
Book an S4 Chemistry tutorTutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.
