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A Level H1 Chemistry Practice Paper 5
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TuitionGoWhere Practice Paper - Chemistry H1 A-Level
TuitionGoWhere Practice Paper (AI)
| Field | Details |
|---|---|
| Subject: | Chemistry H1 (8873) |
| Level: | A-Level |
| Paper: | Practice Paper – Version 5 of 5 |
| Duration: | 2 hours |
| Total Marks: | 80 |
Name: _________________________ Class: _________________________ Date: _________________________
Instructions to Candidates
- This paper consists of three sections: Section A, Section B, and Section C.
- Answer all questions in Section A and Section B.
- Section C consists of two questions. Answer one question only.
- Write your answers in the spaces provided.
- Show all working clearly for calculation questions. Marks are awarded for method.
- You may use a calculator.
- A Data Booklet is provided.
- The number of marks is given in brackets [ ] at the end of each question or part question.
Section A: Multiple Choice (20 marks)
Answer all questions. Circle the correct answer (A, B, C, or D). Each question carries 1 mark.
1. Which statement best describes a Brønsted–Lowry base?
A. A substance that donates a proton. B. A substance that accepts a proton. C. A substance that produces OH⁻ ions in water. D. A substance that accepts an electron pair.
[1]
2. The pH of a 0.010 mol dm⁻³ solution of a monoprotic acid is 3.0. What can be deduced about the acid?
A. It is a strong acid. B. It is a weak acid. C. It is a dilute acid. D. It is a concentrated acid.
[1]
3. Which of the following mixtures would produce a buffer solution with pH less than 7?
A. Excess NaOH(aq) added to CH₃COOH(aq). B. Excess CH₃COOH(aq) added to NaOH(aq). C. Equal volumes of 0.1 mol dm⁻³ HCl(aq) and 0.1 mol dm⁻³ NaCl(aq). D. Equal volumes of 0.1 mol dm⁻³ NH₃(aq) and 0.1 mol dm⁻³ NH₄Cl(aq).
[1]
4. The acid dissociation constant, Ka, for ethanoic acid is 1.8 × 10⁻⁵ mol dm⁻³ at 298 K. What is the pKa of ethanoic acid?
A. 3.74 B. 4.74 C. 5.74 D. 1.8 × 10⁻⁵
[1]
5. Which of the following salts dissolves in water to form an acidic solution?
A. Sodium chloride B. Ammonium chloride C. Potassium ethanoate D. Sodium nitrate
[1]
6. A student titrates 25.0 cm³ of NaOH(aq) with 0.100 mol dm⁻³ HCl(aq). The titre volume is 20.0 cm³. What is the concentration of the NaOH(aq)?
A. 0.0400 mol dm⁻³ B. 0.0800 mol dm⁻³ C. 0.125 mol dm⁻³ D. 0.200 mol dm⁻³
[1]
7. Which indicator would be most suitable for a titration between a weak acid and a strong base?
A. Methyl orange (pH range 3.1–4.4) B. Bromothymol blue (pH range 6.0–7.6) C. Phenolphthalein (pH range 8.3–10.0) D. Litmus (pH range 5.0–8.0)
[1]
8. What is the conjugate base of H₂PO₄⁻?
A. H₃PO₄ B. HPO₄²⁻ C. PO₄³⁻ D. H₂PO₄
[1]
9. A buffer solution contains 0.20 mol dm⁻³ CH₃COOH and 0.20 mol dm⁻³ CH₃COONa. What happens to the pH when a small amount of HCl(aq) is added?
A. The pH increases significantly. B. The pH decreases significantly. C. The pH remains almost unchanged. D. The solution becomes neutral.
[1]
10. Which of the following is NOT a property of a strong acid?
A. It fully dissociates in water. B. It has a high electrical conductivity in aqueous solution. C. It has a large Ka value. D. It has a high pH value.
[1]
11. The pH of pure water at 298 K is 7.0. What is the pH of 0.0010 mol dm⁻³ NaOH(aq) at the same temperature?
A. 3.0 B. 10.0 C. 11.0 D. 12.0
[1]
12. Which equation represents the reaction of aluminium oxide with sodium hydroxide solution?
A. Al₂O₃ + 6NaOH → 2Al(OH)₃ + 3Na₂O B. Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄] C. Al₂O₃ + 2NaOH → 2NaAlO₂ + H₂O D. Al₂O₃ + NaOH → NaAlO₂ + Al(OH)₃
[1]
13. A student prepares a solution by dissolving 0.50 g of NaOH (Mr = 40.0) in water to make 250 cm³ of solution. What is the concentration of the solution?
A. 0.0125 mol dm⁻³ B. 0.0250 mol dm⁻³ C. 0.0500 mol dm⁻³ D. 0.100 mol dm⁻³
[1]
14. The Ka of a weak acid HA is 1.0 × 10⁻⁵ mol dm⁻³. What is the approximate pH of a 0.10 mol dm⁻³ solution of HA?
A. 2.0 B. 3.0 C. 4.0 D. 5.0
[1]
15. Which statement about the equivalence point of a titration is correct?
A. The pH is always 7 at the equivalence point. B. The indicator changes colour at the equivalence point. C. The amounts of acid and base are stoichiometrically equivalent. D. The solution is always neutral at the equivalence point.
[1]
16. A solution has a pH of 4.5. What is the hydrogen ion concentration?
A. 3.2 × 10⁻⁵ mol dm⁻³ B. 3.2 × 10⁻⁴ mol dm⁻³ C. 4.5 × 10⁻⁵ mol dm⁻³ D. 1.0 × 10⁻⁴ mol dm⁻³
[1]
17. Which of the following is an amphoteric oxide?
A. Na₂O B. MgO C. Al₂O₃ D. SO₂
[1]
18. A buffer solution is prepared using a weak acid HA and its sodium salt NaA. Which expression correctly gives the hydrogen ion concentration?
A. [H⁺] = Ka × [HA] / [A⁻] B. [H⁺] = Ka × [A⁻] / [HA] C. [H⁺] = [HA] / (Ka × [A⁻]) D. [H⁺] = Ka / ([HA] × [A⁻])
[1]
19. Which salt is produced when ethanoic acid reacts with potassium hydroxide?
A. Potassium ethanoate B. Potassium ethanol C. Potassium ethanal D. Potassium ethane
[1]
20. What is the ionic product of water, Kw, at 298 K?
A. 1.0 × 10⁻⁷ mol² dm⁻⁶ B. 1.0 × 10⁻¹⁴ mol² dm⁻⁶ C. 1.0 × 10⁻¹⁴ mol dm⁻³ D. 1.0 × 10⁻⁷ mol dm⁻³
[1]
Section B: Structured Questions (40 marks)
Answer all questions in the spaces provided.
21. Carbonic acid, H₂CO₃, is a weak diprotic acid formed when carbon dioxide dissolves in water. It plays an important role in regulating the pH of blood and natural water bodies.
(a) Define the term weak acid. [1]
(b) Write an equation, including state symbols, for the first dissociation of carbonic acid in water. [1]
(c) Write the expression for the first acid dissociation constant, Ka₁, of carbonic acid. [1]
(d) The Ka₁ of carbonic acid is 4.3 × 10⁻⁷ mol dm⁻³ at 298 K. Calculate the pH of a 0.050 mol dm⁻³ solution of carbonic acid, assuming that the second dissociation is negligible. [3]
(e) Explain why the second dissociation of carbonic acid (Ka₂ = 5.6 × 10⁻¹¹ mol dm⁻³) is much weaker than the first dissociation. [2]
22. A student investigates the reaction between magnesium oxide (MgO) and two different acids.
(a) Magnesium oxide is classified as a basic oxide. Write a balanced equation, including state symbols, for the reaction of magnesium oxide with dilute hydrochloric acid. [2]
(b) The student adds 0.40 g of MgO to 50.0 cm³ of 0.50 mol dm⁻³ HCl(aq).
(i) Calculate the amount, in moles, of MgO used. [Mr: MgO = 40.3] [1]
(ii) Calculate the amount, in moles, of HCl used. [1]
(iii) Determine which reagent is in excess. Show your working. [2]
(iv) Calculate the mass of magnesium chloride (MgCl₂) that could be formed. [Mr: MgCl₂ = 95.3] [2]
(c) The student repeats the experiment using 0.40 g of MgO and 50.0 cm³ of 0.50 mol dm⁻³ ethanoic acid (CH₃COOH). The reaction is slower and produces less heat. Explain these observations in terms of the nature of the two acids. [3]
23. A buffer solution is widely used in biochemical and industrial processes to maintain a stable pH.
(a) Explain what is meant by a buffer solution. [2]
(b) A buffer solution is prepared by mixing 30.0 cm³ of 0.20 mol dm⁻³ ethanoic acid (CH₃COOH) with 20.0 cm³ of 0.15 mol dm⁻³ sodium ethanoate (CH₃COONa).
(i) Calculate the concentration of ethanoic acid in the buffer solution after mixing. [1]
(ii) Calculate the concentration of ethanoate ions in the buffer solution after mixing. [1]
(iii) Calculate the pH of this buffer solution. [Ka of ethanoic acid = 1.8 × 10⁻⁵ mol dm⁻³] [3]
(c) A small amount of sodium hydroxide solution is added to the buffer solution in (b). Using equations where appropriate, explain how the buffer solution resists a change in pH. [3]
24. The graph below shows the pH titration curve obtained when 25.0 cm³ of an unknown monoprotic acid, HA, is titrated against 0.100 mol dm⁻³ sodium hydroxide solution.
[The curve shows: initial pH ≈ 3.0, gradual rise, buffer region around pH 4–6, sharp rise from pH 6 to 11, equivalence point at 25.0 cm³ with pH ≈ 8.5, final pH ≈ 12.5]
(a) State whether the acid HA is a strong acid or a weak acid. Explain your answer using evidence from the titration curve. [2]
(b) Determine the concentration of the acid HA. Show your working. [2]
(c) Explain why the pH at the equivalence point is greater than 7. [2]
(d) Suggest a suitable indicator for this titration. Justify your choice. [2]
(e) On the axes below, sketch the pH titration curve that would be obtained if the acid HA were replaced with 25.0 cm³ of 0.100 mol dm⁻³ hydrochloric acid titrated against the same sodium hydroxide solution. Label the equivalence point clearly. [3]
[Axes provided: x-axis "Volume of NaOH(aq) added / cm³" from 0 to 50; y-axis "pH" from 0 to 14]
14 |
|
12 |
|
10 |
|
8 |
|
6 |
|
4 |
|
2 |
|
0 |_____________________________________
0 10 20 30 40 50
Section C: Free Response Questions (20 marks)
Answer one question only from this section. Indicate clearly which question you are answering.
EITHER
25. Acid rain is an environmental problem caused by the dissolution of acidic gases in rainwater. Sulfur dioxide (SO₂) and oxides of nitrogen (NOₓ) from industrial emissions are major contributors.
(a) Sulfur dioxide dissolves in water to form sulfurous acid, H₂SO₃, a weak diprotic acid.
(i) Write an equation for the first dissociation of sulfurous acid in water. [1]
(ii) The Ka₁ of sulfurous acid is 1.5 × 10⁻² mol dm⁻³. Calculate the pH of a 0.010 mol dm⁻³ solution of H₂SO₃, stating any assumption you make. [4]
(b) Unpolluted rainwater has a pH of approximately 5.6 due to dissolved carbon dioxide. A sample of acid rain is found to have a pH of 4.0.
(i) Calculate the ratio of hydrogen ion concentrations in the acid rain compared to unpolluted rainwater. [2]
(ii) Limestone (calcium carbonate) is sometimes added to lakes affected by acid rain to neutralise the acidity. Write a balanced equation, including state symbols, for the reaction between calcium carbonate and sulfuric acid. [2]
(c) Discuss the environmental impact of acid rain on aquatic ecosystems and suggest one method, other than adding limestone, to reduce the formation of acid rain. [3]
OR
26. Carboxylic acids are an important class of organic compounds that exhibit weak acidic behaviour. Ethanoic acid (CH₃COOH) is a common example.
(a) Ethanoic acid is a weak monoprotic acid.
(i) Write an equation, including state symbols, for the dissociation of ethanoic acid in water. [1]
(ii) The Ka of ethanoic acid is 1.8 × 10⁻⁵ mol dm⁻³ at 298 K. Calculate the pH of a 0.10 mol dm⁻³ solution of ethanoic acid. State any assumption you make. [4]
(b) Ethanoic acid reacts with sodium hydroxide in a neutralisation reaction.
(i) Write a balanced equation for this reaction. [1]
(ii) Calculate the volume of 0.20 mol dm⁻³ NaOH(aq) required to neutralise 25.0 cm³ of 0.10 mol dm⁻³ ethanoic acid. [2]
(c) Ethanoic acid is used in the preparation of buffer solutions.
(i) Explain why a mixture of ethanoic acid and sodium ethanoate can act as a buffer solution. [2]
(ii) A buffer solution is prepared by dissolving 0.82 g of sodium ethanoate (Mr = 82.0) in 100 cm³ of 0.10 mol dm⁻³ ethanoic acid. Calculate the pH of this buffer solution. [4]
(d) Compare and contrast the properties of ethanoic acid with those of hydrochloric acid of the same concentration. Include in your answer: relative pH, electrical conductivity, and rate of reaction with magnesium. [3]
END OF PAPER
Answers
TuitionGoWhere Practice Paper - Chemistry H1 A-Level (Answers)
TuitionGoWhere Practice Paper (AI) – Version 5 of 5
Section A: Multiple Choice (20 marks)
| Question | Answer | Explanation |
|---|---|---|
| 1 | B | A Brønsted–Lowry base is defined as a proton (H⁺) acceptor. Option A describes a Brønsted–Lowry acid. Option C describes an Arrhenius base. Option D describes a Lewis base. |
| 2 | B | For a strong monoprotic acid, [H⁺] = concentration of acid. If the acid were strong, pH = –log(0.010) = 2.0. Since the measured pH is 3.0, [H⁺] = 1.0 × 10⁻³ mol dm⁻³, which is less than the acid concentration, indicating partial dissociation – characteristic of a weak acid. |
| 3 | B | A buffer requires a weak acid and its conjugate base in significant amounts. Option B: excess CH₃COOH with NaOH produces a mixture of CH₃COOH (weak acid) and CH₃COONa (conjugate base). The pH of an acidic buffer is less than 7. Option D produces a basic buffer (pH > 7). |
| 4 | B | pKa = –log₁₀(Ka) = –log₁₀(1.8 × 10⁻⁵) = 4.74 |
| 5 | B | NH₄Cl dissolves to give NH₄⁺ and Cl⁻. NH₄⁺ is the conjugate acid of the weak base NH₃ and undergoes hydrolysis: NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺, producing an acidic solution. The other salts are from strong acid-strong base (neutral) or strong base-weak acid (basic). |
| 6 | B | n(HCl) = 0.100 × 20.0/1000 = 0.00200 mol. NaOH + HCl → NaCl + H₂O (1:1 ratio). n(NaOH) = 0.00200 mol. c(NaOH) = 0.00200 / (25.0/1000) = 0.0800 mol dm⁻³. |
| 7 | C | Weak acid-strong base titrations have an equivalence point pH > 7 (typically 8–10). Phenolphthalein changes colour in the pH range 8.3–10.0, which brackets the equivalence point. Methyl orange changes at pH 3.1–4.4, which is too acidic. |
| 8 | B | The conjugate base of an acid is the species formed when the acid donates one proton. H₂PO₄⁻ → HPO₄²⁻ + H⁺. Therefore, HPO₄²⁻ is the conjugate base. |
| 9 | C | The buffer contains CH₃COOH and CH₃COO⁻. Added H⁺ reacts with CH₃COO⁻: CH₃COO⁻ + H⁺ → CH₃COOH. The [H⁺] remains almost unchanged, so pH remains almost constant. |
| 10 | D | A strong acid fully dissociates, producing a high [H⁺], which means a low pH, not a high pH. All other options are correct properties of strong acids. |
| 11 | C | NaOH is a strong base: [OH⁻] = 0.0010 mol dm⁻³. pOH = –log(0.0010) = 3.0. pH = 14 – pOH = 14 – 3.0 = 11.0. |
| 12 | B | Al₂O₃ is amphoteric. With NaOH, it forms sodium aluminate: Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄]. This is the correct balanced equation for the reaction in aqueous solution. |
| 13 | C | n(NaOH) = 0.50 / 40.0 = 0.0125 mol. Volume = 250 cm³ = 0.250 dm³. c = 0.0125 / 0.250 = 0.0500 mol dm⁻³. |
| 14 | B | For a weak acid: [H⁺] = √(Ka × c) = √(1.0 × 10⁻⁵ × 0.10) = √(1.0 × 10⁻⁶) = 1.0 × 10⁻³ mol dm⁻³. pH = –log(1.0 × 10⁻³) = 3.0. |
| 15 | C | The equivalence point is when the amounts of acid and base are stoichiometrically equivalent (moles of H⁺ = moles of OH⁻). The pH at equivalence depends on the strengths of the acid and base. The indicator end point should coincide with the equivalence point, but they are not the same concept. |
| 16 | A | [H⁺] = 10⁻ᵖᴴ = 10⁻⁴·⁵ = 3.16 × 10⁻⁵ ≈ 3.2 × 10⁻⁵ mol dm⁻³. |
| 17 | C | Al₂O₃ is amphoteric – it reacts with both acids and bases. Na₂O and MgO are basic oxides. SO₂ is an acidic oxide. |
| 18 | A | From Ka = [H⁺][A⁻]/[HA], rearranging: [H⁺] = Ka × [HA]/[A⁻]. This is the Henderson-Hasselbalch relationship. |
| 19 | A | CH₃COOH + KOH → CH₃COOK + H₂O. The salt is potassium ethanoate. |
| 20 | B | Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴ mol² dm⁻⁶ at 298 K. The units are mol² dm⁻⁶ because it is the product of two concentrations. |
Section B: Structured Questions (40 marks)
Question 21
(a) A weak acid is an acid that partially dissociates in water, existing in equilibrium with its ions. [1]
(b) H₂CO₃(aq) ⇌ HCO₃⁻(aq) + H⁺(aq) [1] [Award 1 mark for correct equation with equilibrium arrow and state symbols. Accept H₂CO₃(aq) + H₂O(l) ⇌ HCO₃⁻(aq) + H₃O⁺(aq)]
(c) Ka₁ = [HCO₃⁻][H⁺] / [H₂CO₃] [1] [Award 1 mark for correct expression. Do not penalise if [H₃O⁺] is used instead of [H⁺]. Do not accept inclusion of [H₂O].]
(d) H₂CO₃ ⇌ H⁺ + HCO₃⁻ Let [H⁺] = x mol dm⁻³ [1] Ka₁ = x² / (0.050 – x) ≈ x² / 0.050 (since Ka is very small, x << 0.050) [1] x² = 4.3 × 10⁻⁷ × 0.050 = 2.15 × 10⁻⁸ x = √(2.15 × 10⁻⁸) = 1.47 × 10⁻⁴ mol dm⁻³ [1] pH = –log₁₀(1.47 × 10⁻⁴) = 3.83 [1]
[Award marks as follows: 1 mark for setting up Ka expression; 1 mark for stating/using the assumption; 1 mark for correct [H⁺]; 1 mark for correct pH. Accept 3.83 or 3.8.]
(e) The first dissociation involves removing H⁺ from a neutral H₂CO₃ molecule. [1] The second dissociation involves removing H⁺ from the negatively charged HCO₃⁻ ion. It is more difficult to remove a positively charged proton from a negatively charged ion due to stronger electrostatic attraction. Therefore, Ka₂ is much smaller than Ka₁. [1]
[Total: 8 marks]
Question 22
(a) MgO(s) + 2HCl(aq) → MgCl₂(aq) + H₂O(l) [2] [1 mark for correct formulae; 1 mark for correct state symbols and balancing]
(b)(i) n(MgO) = 0.40 / 40.3 = 0.00993 mol ≈ 0.0099 mol [1]
(b)(ii) n(HCl) = 0.50 × 50.0/1000 = 0.0250 mol [1]
(b)(iii) From equation: 1 mol MgO reacts with 2 mol HCl. 0.00993 mol MgO requires 0.00993 × 2 = 0.01986 mol HCl. [1] Available HCl = 0.0250 mol, which is greater than 0.01986 mol. Therefore, HCl is in excess. [1]
(b)(iv) MgO is the limiting reagent. n(MgCl₂) = n(MgO) = 0.00993 mol [1] Mass of MgCl₂ = 0.00993 × 95.3 = 0.946 g ≈ 0.95 g [1]
(c) HCl is a strong acid that fully dissociates, giving a high concentration of H⁺ ions. [1] Ethanoic acid is a weak acid that partially dissociates, giving a lower concentration of H⁺ ions. [1] The lower [H⁺] in ethanoic acid means fewer effective collisions per unit time (slower rate) and less heat released as the acid dissociates during reaction. [1]
[Total: 10 marks]
Question 23
(a) A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. [1] It consists of a weak acid and its conjugate base (or a weak base and its conjugate acid) in significant concentrations. [1]
(b)(i) Total volume = 30.0 + 20.0 = 50.0 cm³ [CH₃COOH] = (0.20 × 30.0/1000) / (50.0/1000) = 0.12 mol dm⁻³ [1]
(b)(ii) [CH₃COO⁻] = (0.15 × 20.0/1000) / (50.0/1000) = 0.060 mol dm⁻³ [1]
(b)(iii) [H⁺] = Ka × [CH₃COOH] / [CH₃COO⁻] [1] [H⁺] = (1.8 × 10⁻⁵) × (0.12 / 0.060) = 3.6 × 10⁻⁵ mol dm⁻³ [1] pH = –log₁₀(3.6 × 10⁻⁵) = 4.44 [1]
(c) The buffer contains CH₃COOH (weak acid) and CH₃COO⁻ (conjugate base). [1] When OH⁻ is added: OH⁻ + CH₃COOH → CH₃COO⁻ + H₂O. The added OH⁻ is neutralised by the weak acid. [1] The ratio [CH₃COOH]/[CH₃COO⁻] changes only slightly, so [H⁺] and pH remain almost constant. [1]
[Total: 10 marks]
Question 24
(a) HA is a weak acid. [1] Evidence: The initial pH is approximately 3.0, which is higher than expected for a strong acid of similar concentration (if HA were 0.10 mol dm⁻³ strong acid, pH would be 1.0). Also, the equivalence point pH is approximately 8.5 (>7), indicating the salt undergoes hydrolysis, characteristic of a weak acid-strong base titration. [1]
(b) At equivalence point: n(NaOH) = 0.100 × 25.0/1000 = 0.00250 mol [1] Since HA is monoprotic, n(HA) = 0.00250 mol. c(HA) = 0.00250 / (25.0/1000) = 0.100 mol dm⁻³ [1]
(c) At the equivalence point, all the weak acid HA has been converted to its conjugate base A⁻. [1] A⁻ undergoes hydrolysis: A⁻ + H₂O ⇌ HA + OH⁻, producing OH⁻ ions and making the solution alkaline (pH > 7). [1]
(d) Phenolphthalein (pH range 8.3–10.0). [1] The equivalence point pH is approximately 8.5, which falls within the colour change range of phenolphthalein, ensuring a sharp colour change at the end point. [1]
(e) The curve should show:
- Initial pH ≈ 1.0 (for 0.100 mol dm⁻³ strong acid) [1]
- Gradual rise, then very sharp vertical rise from pH ~3 to ~11 around 25.0 cm³ [1]
- Equivalence point at 25.0 cm³, pH = 7.0 (clearly labelled) [1]
- Final pH approaching ~12.5
[Total: 12 marks]
Section C: Free Response Questions (20 marks)
Question 25
(a)(i) H₂SO₃(aq) ⇌ HSO₃⁻(aq) + H⁺(aq) [1] [Accept H₂SO₃(aq) + H₂O(l) ⇌ HSO₃⁻(aq) + H₃O⁺(aq)]
(a)(ii) H₂SO₃ ⇌ H⁺ + HSO₃⁻ Let [H⁺] = x mol dm⁻³ [1] Ka₁ = [H⁺][HSO₃⁻] / [H₂SO₃] = x² / (0.010 – x) [1] Since Ka₁ is relatively large (1.5 × 10⁻²), the assumption x << 0.010 may not be valid. However, using the quadratic formula or approximation: If we assume x << 0.010: x² = 1.5 × 10⁻² × 0.010 = 1.5 × 10⁻⁴; x = 0.0122 mol dm⁻³ (which is >0.010, so assumption invalid). [1] Solving quadratic: x² + 1.5 × 10⁻²x – 1.5 × 10⁻⁴ = 0 x = [–0.015 + √(0.015² + 4 × 1.5 × 10⁻⁴)] / 2 = [–0.015 + √(2.25 × 10⁻⁴ + 6.0 × 10⁻⁴)] / 2 x = [–0.015 + √(8.25 × 10⁻⁴)] / 2 = [–0.015 + 0.0287] / 2 = 0.00685 mol dm⁻³ [1] pH = –log₁₀(0.00685) = 2.16 [1]
[Award marks: 1 for Ka expression; 1 for recognising assumption may be invalid; 1 for correct method (quadratic or successive approximation); 1 for correct pH. Accept 2.16 or 2.2.]
(b)(i) Acid rain: [H⁺] = 10⁻⁴·⁰ = 1.0 × 10⁻⁴ mol dm⁻³ [1] Unpolluted rain: [H⁺] = 10⁻⁵·⁶ = 2.51 × 10⁻⁶ mol dm⁻³ Ratio = (1.0 × 10⁻⁴) / (2.51 × 10⁻⁶) = 39.8 ≈ 40 [1] [The acid rain has approximately 40 times the hydrogen ion concentration of unpolluted rainwater.]
(b)(ii) CaCO₃(s) + H₂SO₄(aq) → CaSO₄(aq) + H₂O(l) + CO₂(g) [2] [1 mark for correct formulae; 1 mark for correct state symbols and balancing]
(c) Environmental impacts:
- Lowered pH of lakes and rivers kills fish and other aquatic organisms [1]
- Mobilises toxic metal ions (e.g., Al³⁺) from soils, which damage fish gills [1]
- Disrupts aquatic food chains and reduces biodiversity
Method to reduce acid rain formation:
- Install scrubbers in factory chimneys to remove SO₂ from emissions before release (e.g., using limestone slurry or alkaline solutions) [1] [Accept other valid methods: catalytic converters to reduce NOₓ emissions, use of low-sulfur fuels, alternative energy sources]
[Total: 20 marks]
Question 26
(a)(i) CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq) [1] [Accept CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)]
(a)(ii) CH₃COOH ⇌ CH₃COO⁻ + H⁺ Let [H⁺] = x mol dm⁻³ [1] Ka = [CH₃COO⁻][H⁺] / [CH₃COOH] = x² / (0.10 – x) [1] Assumption: x << 0.10 since Ka is very small (1.8 × 10⁻⁵). [1] x² ≈ 1.8 × 10⁻⁵ × 0.10 = 1.8 × 10⁻⁶ x = √(1.8 × 10⁻⁶) = 1.34 × 10⁻³ mol dm⁻³ pH = –log₁₀(1.34 × 10⁻³) = 2.87 [1]
[Award marks: 1 for Ka expression; 1 for stating assumption; 1 for correct [H⁺]; 1 for correct pH. Accept 2.87 or 2.9.]
(b)(i) CH₃COOH(aq) + NaOH(aq) → CH₃COONa(aq) + H₂O(l) [1]
(b)(ii) n(CH₃COOH) = 0.10 × 25.0/1000 = 0.00250 mol [1] From equation, n(NaOH) = 0.00250 mol. Volume of NaOH = 0.00250 / 0.20 = 0.0125 dm³ = 12.5 cm³ [1]
(c)(i) The mixture contains a weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻ from CH₃COONa). [1] Added H⁺ reacts with CH₃COO⁻: CH₃COO⁻ + H⁺ → CH₃COOH. Added OH⁻ reacts with CH₃COOH: CH₃COOH + OH⁻ → CH₃COO⁻ + H₂O. In both cases, the pH remains almost unchanged. [1]
(c)(ii) n(CH₃COONa) = 0.82 / 82.0 = 0.0100 mol [1] Volume of solution = 100 cm³ = 0.100 dm³ [CH₃COO⁻] = 0.0100 / 0.100 = 0.100 mol dm⁻³ [1] [CH₃COOH] = 0.10 mol dm⁻³ [H⁺] = Ka × [CH₃COOH] / [CH₃COO⁻] = (1.8 × 10⁻⁵) × (0.10 / 0.100) = 1.8 × 10⁻⁵ mol dm⁻³ [1] pH = –log₁₀(1.8 × 10⁻⁵) = 4.74 [1]
(d) Comparison of 0.10 mol dm⁻³ CH₃COOH and 0.10 mol dm⁻³ HCl:
| Property | CH₃COOH | HCl | Explanation |
|---|---|---|---|
| pH | ~2.9 | ~1.0 | HCl fully dissociates; CH₃COOH partially dissociates, giving lower [H⁺] |
| Electrical conductivity | Lower | Higher | Fewer ions in CH₃COOH solution due to partial dissociation |
| Rate of reaction with Mg | Slower | Faster | Lower [H⁺] in CH₃COOH means fewer effective collisions per unit time |
[3 marks – 1 for each correct comparison with explanation]
[Total: 20 marks]
END OF ANSWER KEY
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