R3.1 Exam Practice
Acids & Bases
Section B: Data Analysis (Paper 1B Style)
Calculator and Data Booklet permitted. Show all working clearly.
Question 1: Comparing Acid Strength Explain
4 marksA student measures the pH of three solutions at 25 °C:
| Solution | Concentration / mol dm⁻³ | pH |
|---|---|---|
| A: HCl(aq) | 0.0100 | 2.00 |
| B: CH₃COOH(aq) | 0.0100 | 3.38 |
| C: NaOH(aq) | 0.0100 | 12.00 |
(a) Identify which solutions are acidic and which are basic. [1]
(b) Explain why Solution B has a higher pH than Solution A despite having the same concentration. [2]
(c) Calculate [H⁺] for Solution C. [1]
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(a) A and B are acidic (pH < 7); C is basic (pH > 7) [1]
(b)
- HCl is a strong acid; fully ionises, so [H⁺] = 0.0100 mol dm⁻³ [1]
- CH₃COOH is a weak acid; partially ionises, producing a lower [H⁺] and therefore a higher pH [1]
(c) [H⁺] = Kw/[OH⁻] = 1.00 × 10⁻¹⁴/0.0100 = 1.00 × 10⁻¹² mol dm⁻³ [1]
Question 2: Conjugate Acid-Base Pairs Identify
4 marksEthanoic acid establishes the following equilibrium in water:
CH₃COOH(aq) + H₂O(l) ⇌ CH₃COO⁻(aq) + H₃O⁺(aq)
(a) Identify the two conjugate acid-base pairs in this equilibrium. [2]
(b) Explain what is meant by the term amphiprotic, using water as an example. [2]
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(a)
- Pair 1: CH₃COOH (acid) and CH₃COO⁻ (conjugate base) [1]
- Pair 2: H₂O (base) and H₃O⁺ (conjugate acid) [1]
(b)
- Amphiprotic = can both donate and accept a proton (H⁺) [1]
- Water can donate H⁺ to form OH⁻ or accept H⁺ to form H₃O⁺ [1]
Section C: Structured Questions (Paper 2 Style)
Show all working. State answers with appropriate significant figures and units.
Question 3: Acid-Base Fundamentals Define
5 marks(a) Define a Brønsted-Lowry acid and a Brønsted-Lowry base. [2]
(b) Distinguish between a strong acid and a weak acid in terms of degree of ionisation. [2]
(c) State one property that distinguishes a "concentrated" acid from a "strong" acid. [1]
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(a) Brønsted-Lowry acid = proton (H⁺) donor [1]; Brønsted-Lowry base = proton (H⁺) acceptor [1]
(b) Strong acid = complete (~100%) ionisation in water [1]; Weak acid = partial ionisation (equilibrium lies to the left) [1]
(c) Concentration refers to moles of solute per dm³; strength refers to the proportion of molecules that ionise [1]
Question 4: pH and Kw Calculations Deduce
4 marks(a) State the formula used for calculating pH. [1]
(b) Calculate the pH of 0.0500 mol dm⁻³ HNO₃. [1]
(c) Deduce the concentration of OH⁻ ions in a solution of pH 3 at 25 °C, given Kw = 1.00 × 10⁻¹⁴. [2]
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(a) pH = -log₁₀[H⁺] [1]
(b) HNO₃ is a strong acid; pH = -log(0.0500) = 1.30 [1]
(c)
- [H⁺] = 10⁻³ = 1.00 × 10⁻³ mol dm⁻³ [1]
- [OH⁻] = Kw/[H⁺] = 1.00 × 10⁻¹⁴/1.00 × 10⁻³ = 1.00 × 10⁻¹¹ mol dm⁻³ [1]
Question 5: Acid Deposition Explain
5 marks(a) State two pollutants that cause acid deposition. [1]
(b) Explain how SO₂ leads to the formation of acid rain. Include a balanced equation. [2]
(c) Suggest two environmental effects of acid deposition. [2]
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(a) SO₂ and NOx (nitrogen oxides) [1] (Both required)
(b)
- SO₂ dissolves in rainwater / is oxidised to SO₃ [1]
- SO_2(g) + H_2O(l) → H_2SO_3(aq) OR SO_3(g) + H_2O(l) → H_2SO_4(aq) [1]
(c) Any two of: acidification of lakes kills aquatic life [1]; corrosion of limestone buildings / statues [1]; leaching minerals from soil / releasing toxic Al³⁺ ions; damage to forests / vegetation
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