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S4 Chemistry Acids and Alkalis, Paper 1: pH, titration and neutralisation heat
S4 Chemistry Acids and Alkalis, Paper 1: pH, titration and neutralisation heat
Ten graded items moving from pH and hydrogen ion concentration to one-to-one and diprotic titrations and finally an enthalpy calculation.
Focus Write the mole ratio first, double the hydrogen ions for a diprotic acid, and divide m c delta T by the moles of alkali.
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)
- FoundationWhat is the hydrogen ion concentration of a solution with pH 2, in mol per cubic decimetre?
- FoundationHydrochloric acid is a strong acid that ionises completely. Find the pH of a 0.010 mol dm^-3 solution.
- Foundation25.0 cm^3 of 0.100 mol dm^-3 sodium hydroxide is neutralised by 0.100 mol dm^-3 hydrochloric acid. What volume is needed?
- FoundationA solution of pH 3 is diluted a hundred times. What is the new pH?
- Intermediate20.0 cm^3 of 0.150 mol dm^-3 sodium hydroxide is titrated with 0.100 mol dm^-3 sulphuric acid. Find the volume at the equivalence point.
- Intermediate50.0 cm^3 of 1.00 mol dm^-3 hydrochloric acid is mixed with 50.0 cm^3 of 1.00 mol dm^-3 sodium hydroxide and the temperature rises by 6.7 degrees. Find the enthalpy of neutralisation, taking density 1.0 g cm^-3 and specific heat capacity 4.2 J g^-1 K^-1.
- IntermediateA monobasic acid of 25.0 cm^3 is titrated with 0.100 mol dm^-3 sodium hydroxide and the titres are 24.8, 25.2 and 25.0 cm^3. Find the mean titre and the acid concentration.
- Challenge25.0 cm^3 of 0.100 mol dm^-3 sodium carbonate is fully neutralised by 0.100 mol dm^-3 hydrochloric acid. Find the volume of acid.
- Challenge20.0 cm^3 of 0.050 mol dm^-3 sulphuric acid is titrated with 0.100 mol dm^-3 sodium hydroxide. Find the volume at the equivalence point.
- Error redoA student sees pH 0 and says there are no hydrogen ions at all. Give the correct concentration.
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 1 x 10^-2 mol dm^-3 | pH two means the hydrogen ion concentration is one times ten to the minus two moles per cubic decimetre. |
| 2 | pH 2.0 | A strong acid ionises completely, so the pH is the negative logarithm of 0.010, which is 2.0. |
| 3 | 25.0 cm^3 | Sodium hydroxide and hydrochloric acid react one to one, so equal concentrations need equal volumes. |
| 4 | pH 5 | Diluting ten times raises pH by one, so a hundredfold dilution raises it by two. |
| 5 | 15.0 cm^3 | The alkali holds 0.0030 mol, and sulphuric acid is diprotic, so 0.0015 mol is enough, which is 15.0 cm^3. |
| 6 | About 56.3 kJ mol^-1 | The mixture releases 100 times 4.2 times 6.7 joules, and dividing by 0.050 mol gives about 56.3 kilojoules per mole. |
| 7 | Mean titre 25.0 cm^3 and acid concentration 0.100 mol dm^-3 | The three titres agree within 0.2 cm^3, so the mean is 25.0 cm^3 and a monobasic acid then has the same concentration as the alkali. |
| 8 | 50.0 cm^3 | Each carbonate ion takes two hydrogen ions, so 0.0025 mol of the salt needs 0.0050 mol of acid, which is 50.0 cm^3. |
| 9 | 20.0 cm^3 | The acid gives 0.0020 mol of hydrogen ions, and the alkali supplies one ion each, so 20.0 cm^3 is needed. |
| 10 | 1.0 mol dm^-3 | pH zero means a concentration of one mole per cubic decimetre, and pH can fall below zero, so the ions are never absent. |
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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