中文（繁體）
中文（简体）
English
S5 Chemistry Titration and pH, Paper 3: diprotic acids, dilution, mixtures and titration data
S5 Chemistry Titration and pH, Paper 3: diprotic acids, dilution, mixtures and titration data
Ten graded items from a carbonate volume through diprotic and diluted pH, weak acid Ka, back titration and a mean titre, each with an answer.
Focus Carbonate reacts one to two with acid; take the square root for a weak acid; subtract the excess in a back titration; discard the odd reading.
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: S5 Chemistry: Acid-Base Titration and pH |
| Level | Four basic, three advanced, two challenge and one error-repair item |
02 Questions (10 items)
- FoundationWhat volume of 0.0500 mol dm^-3 sodium carbonate neutralises 20.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid?
- FoundationWhat is the pH of 0.0025 mol dm^-3 sulphuric acid, fully ionised in both stages?
- FoundationWhat is the pH of 0.0020 mol dm^-3 sodium hydroxide at 25 C?
- FoundationWhich indicator is chosen when hydrochloric acid titrates aqueous ammonia, whose equivalence point is below pH 7?
- IntermediateWhat is the approximate pH of 0.100 mol dm^-3 of a weak acid HA with Ka = 1.8 x 10^-5?
- Intermediate10.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid is diluted to 250 cm^3. What is the new pH?
- Intermediate30.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid is mixed with 20.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide. What is the pH?
- Challenge1.00 g of a sample containing calcium carbonate is treated with 50.0 cm^3 of 0.200 mol dm^-3 hydrochloric acid and the excess needs 20.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide. Find the percentage by mass of calcium carbonate.
- ChallengeTitres of 23.90, 24.00 and 27.50 cm^3 are obtained against 25.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide. Find the acid concentration.
- Error redoError repair: a pipette rinsed with distilled water is used straight away to measure the sodium hydroxide. Does the calculated acid concentration come out high or low?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 20.0 cm^3 | The acid is 0.00200 mol and sodium carbonate reacts one to two, so 0.00100 mol is needed, which divides by 0.0500 to give 20.0 cm^3. |
| 2 | pH 2.30 | The hydrogen ion concentration is 0.0025 times two, or 0.0050, whose negative logarithm is 2.30. |
| 3 | pH 11.30 | The hydroxide concentration is 0.0020, so the pOH is 2.70 and the pH is 14 minus 2.70. |
| 4 | Methyl orange | At the equivalence point only the salt of a weak base remains and ammonium ions hydrolyse to make it acidic, which methyl orange covers from pH 3.1 to 4.4. |
| 5 | pH 2.87 | For a weak acid the hydrogen ion concentration is about the square root of Ka times the concentration, giving 1.3 x 10^-3. |
| 6 | pH 2.40 | Diluting 25 times takes the hydrogen ion concentration from 0.100 down to 0.00400, whose negative logarithm is 2.40. |
| 7 | pH 1.70 | The acid is in excess by 0.00100 mol, which over the 0.0500 dm^3 total gives 0.0200 mol dm^-3 of hydrogen ions. |
| 8 | 40.0% | The acid totals 0.0100 mol and the back titration uses 0.00200 mol, so 0.00800 mol reacted, which is 0.00400 mol or 0.400 g of calcium carbonate. |
| 9 | 0.104 mol dm^-3 | The 27.50 cm^3 reading is far off and discarded, so the mean of 23.90 and 24.00 is 23.95 cm^3, and 0.00250 mol over 0.02395 dm^3 gives the answer. |
| 10 | Low, because the alkali being titrated is diluted | Water left in the pipette dilutes the alkali, so fewer moles are transferred, less acid is needed and the calculated concentration comes out low. |
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 S5 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.
