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S4 Chemistry Acids and Alkalis, Paper 4: indicators, dilution and titration means
S4 Chemistry Acids and Alkalis, Paper 4: indicators, dilution and titration means
Ten graded items combining titration data, dilution factors and enthalpy of neutralisation.
Focus Discard wild readings before averaging a titration; dilution keeps the moles constant; enthalpy divides joules by the moles of water formed.
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 0.0010 mol dm^-3 sulphuric acid, a fully ionised diprotic acid.
- FoundationFind the pH at 25 degrees of 0.050 mol dm^-3 sodium hydroxide.
- Foundation25.0 cm^3 of 0.100 mol dm^-3 hydrochloric acid is titrated with 0.200 mol dm^-3 potassium hydroxide. Find the volume needed.
- FoundationWhich indicator suits a titration of a strong acid with a weak alkali?
- Intermediate20.0 cm^3 of 0.50 mol dm^-3 hydrochloric acid is diluted to 250 cm^3. Find the pH.
- Intermediate50.0 cm^3 of 0.50 mol dm^-3 hydrochloric acid is mixed with 50.0 cm^3 of 0.50 mol dm^-3 sodium hydroxide and the temperature rises by 3.35 degrees. Find the enthalpy of neutralisation, taking the specific heat capacity as 4.2 J g^-1 K^-1 and the density as 1.0 g cm^-3.
- IntermediateThree titrations of 25.0 cm^3 portions of an acid use 23.50, 23.60 and 23.40 cm^3 of 0.100 mol dm^-3 sodium hydroxide. Find the acid concentration.
- Challenge40.0 cm^3 of 0.050 mol dm^-3 hydrochloric acid is mixed with 60.0 cm^3 of 0.050 mol dm^-3 sodium hydroxide. Find the pH.
- ChallengeA 0.20 mol dm^-3 solution of a weak acid HA has pH 2.50. Find Ka.
- Error redoA student says diluting a weak acid tenfold must raise the pH by exactly one unit. Give the correct statement.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | pH 2.70 | Each acid molecule releases two hydrogen ions, so the concentration is 0.0020 and the pH is 2.70. |
| 2 | pH 12.70 | The hydroxide concentration is 0.050 so the pOH is 1.30, and pH plus pOH equals 14 at 25 degrees. |
| 3 | 12.5 cm^3 | The acid is 0.0025 mol and reacts one to one, so 0.0025 divided by 0.200 gives the volume. |
| 4 | Methyl orange | The equivalence point lies in the acidic range, where methyl orange changes from red to yellow near pH 3 to 4. |
| 5 | pH 1.40 | The moles of acid stay at 0.010, so the new concentration is 0.040 and the pH is 1.40. |
| 6 | 56.3 kJ mol^-1 | The heat released is 100 times 4.2 times 3.35, or 1 407 joules, for 0.025 mol of water formed. |
| 7 | 0.0940 mol dm^-3 | The mean volume is 23.50 cm^3 or 0.00235 mol, which divided by 0.0250 dm^3 gives 0.0940. |
| 8 | pH 12.0 | The alkali gives 3.0 mmol against 2.0 mmol of acid, leaving 1.0 mmol of hydroxide in 0.100 dm^3 so the pOH is 2.0. |
| 9 | About 5.0 x 10^-5 mol dm^-3 | The hydrogen ion concentration is 3.16 times 10 to the minus 3, and squaring it over 0.20 gives 5.0 times 10 to the minus 5. |
| 10 | The pH rises by only about 0.5 because more molecules ionise on dilution | Only a fully ionised strong acid rises by exactly one unit; a weak acid ionises further on dilution, so the rise is about half a unit. |
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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