These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. Data for every question: Kw = 1.0 × 10⁻¹⁴ at 25 °C; Ka(CH₃COOH) = 1.8 × 10⁻⁵; Ka(NH₄⁺) = 5.6 × 10⁻¹⁰.
Question 1 (multiple choice · foundation)
Which list contains only strong acids?
- (A) HCl, HNO₃, HClO₄
- (B) HF, HCl, HBr
- (C) HNO₃, CH₃COOH, H₂SO₄
- (D) HI, HOCl, HBr
Answer and explanation
Answer: (A). HCl, HNO₃ and HClO₄ are all on the list of strong acids, which ionise completely in water.
- (B) HF is a weak acid, even though fluorine is the most electronegative element.
- (C) CH₃COOH is a carboxylic acid, the standard example of a weak acid.
- (D) HOCl has only one O atom on the chlorine and is a weak acid; compare HClO₄, with four.
Question 2 (multiple choice · core)
Phosphorous acid has the formula H₃PO₃. Its structure has a central phosphorus atom bonded to one H atom, one O atom (by a double bond) and two O–H groups. How many protons can one H₃PO₃ molecule donate in reactions with a strong base?
- (A) 0
- (B) 1
- (C) 2
- (D) 3
Answer and explanation
Answer: (C). In this molecule, only the hydrogens bonded to oxygen are acidic. H₃PO₃ has two O–H groups, so it is diprotic. The H bonded directly to P is not acidic: the P–H bond is barely polar and a negative charge on P would not be stabilised.
- (A) ignores the O–H groups, which behave like those in other oxoacids.
- (B) counts only one O–H group.
- (D) counts every H in the formula; the formula alone does not tell you which hydrogens are acidic.
Question 3 (multiple choice · core)
Which of these is the strongest acid?
- (A) CH₃CH₂OH
- (B) CH₃COOH
- (C) ClCH₂COOH
- (D) Cl₃CCOOH
Answer and explanation
Answer: (D). All of (B)–(D) have a carboxylate conjugate base stabilised by resonance. In (D), three Cl atoms on the neighbouring carbon pull electron density away from the carboxylate group (inductive effect), spreading out the negative charge most. The most stable conjugate base belongs to the strongest acid.
- (A) Ethanol’s conjugate base has its charge on one O atom with no resonance, so it is by far the weakest acid here.
- (B) Acetic acid has no electronegative substituent.
- (C) One Cl atom has a smaller inductive effect than three.
Question 4 (multiple choice · core)
Which species is the strongest base in water?
- (A) Cl⁻
- (B) NO₃⁻
- (C) CH₃COO⁻
- (D) NH₃
Answer and explanation
Answer: (D). Use Kb = Kw ÷ Ka of the conjugate acid. NH₃: Kb = 1.0 × 10⁻¹⁴ ÷ 5.6 × 10⁻¹⁰ = 1.8 × 10⁻⁵. CH₃COO⁻: Kb = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰. Ammonia’s Kb is much larger.
- (A) and (B) Cl⁻ and NO₃⁻ are conjugate bases of strong acids, so they have essentially no basic behaviour in water.
- (C) Acetate is a weak base, but much weaker than ammonia, because resonance spreads its charge over two O atoms.
Question 5 (constructed response · core)
Nitrous acid, HNO₂, is a weak acid. Nitric acid, HNO₃, is a strong acid. In both, the acidic hydrogen is bonded to an oxygen atom, which is bonded to nitrogen.
(a) Describe how the negative charge is distributed in the conjugate bases NO₂⁻ and NO₃⁻. You may refer to resonance structures. (b) Use your answer to (a) to explain why HNO₃ is the stronger acid. (c) Predict which is the stronger acid, H₂SO₃ or H₂SO₄, and justify your prediction with one structural reason.
Worked solution and suggested Marlbridge rubric
(a) NO₂⁻ has two equivalent resonance structures, so its negative charge is shared between two O atoms. NO₃⁻ has three equivalent resonance structures, so its charge is shared between three O atoms.
(b) A charge spread over more atoms is more stable. NO₃⁻ is therefore more stable than NO₂⁻ and has less tendency to take back a proton. The extra O atom on nitrogen in HNO₃ also pulls electron density away from the O–H bond (inductive effect). So HNO₃ ionises completely and HNO₂ only partly.
(c) H₂SO₄ is stronger. It has one more O atom bonded to the sulfur. That extra O pulls electron density away from the O–H groups and lets the conjugate base, HSO₄⁻, spread its charge over more O atoms than HSO₃⁻ can.
| Point | What earns it |
|---|---|
| 1 | Charge in NO₂⁻ shared by two O atoms and in NO₃⁻ by three, through resonance |
| 1 | Links greater charge delocalisation to a more stable conjugate base and so a stronger acid |
| 1 | Predicts H₂SO₄ as the stronger acid |
| 1 | Justifies with more O atoms on S (inductive effect, or more resonance in the conjugate base) |
Do not award the point in (b) for “HNO₃ has more oxygen” without saying what this does to the conjugate base.
Question 6 (constructed response · core)
(a) Explain, at the level of particles, why aqueous sodium hydroxide is a strong base but aqueous ammonia is a weak base. (b) Calculate Kb for the acetate ion and state whether acetate is a weak or a strong base. (c) A student writes: “Acetate is basic because acetic acid is a weak acid. So chloride must also be basic, because HCl is an acid too.” Evaluate this claim.
Worked solution and suggested Marlbridge rubric
(a) NaOH is an ionic compound that dissociates completely in water, releasing OH⁻ ions directly: every formula unit gives one OH⁻. Ammonia contains no hydroxide. It makes OH⁻ only by accepting a proton from water through its nitrogen lone pair, NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, and this equilibrium lies to the left, so only a small fraction of NH₃ reacts.
(b) Kb = Kw ÷ Ka = 1.0 × 10⁻¹⁴ ÷ 1.8 × 10⁻⁵ = 5.6 × 10⁻¹⁰. This is much less than 1, so acetate is a weak base.
(c) The claim is incorrect for chloride. The basicity of a conjugate base depends on the strength of its acid. Acetic acid is weak, so acetate takes protons back to a measurable extent (Kb = 5.6 × 10⁻¹⁰). HCl is a strong acid: Cl⁻ is a large, stable ion with no tendency to take a proton back in water, so Cl⁻ has essentially no basic behaviour and does not change the pH.
| Point | What earns it |
|---|---|
| 1 | NaOH dissociates completely to give OH⁻ directly |
| 1 | NH₃ accepts a proton from water using its lone pair; the equilibrium lies to the left |
| 1 | Kb = 5.6 × 10⁻¹⁰ with working, classified as weak |
| 1 | States the claim fails for Cl⁻ because HCl is a strong acid, so its conjugate base is negligibly weak |
Question 7 (argumentation · stretch)
A student makes two claims:
- Claim 1: “Across period 2, the acid strength of the hydrogen compounds increases in the order CH₄ < NH₃ < H₂O < HF, because the electronegativity of the atom bonded to H increases.”
- Claim 2: “So, down group 17, HF must be the strongest of the hydrogen halides, because fluorine is the most electronegative halogen.”
(a) Evaluate Claim 1, using the stability of the conjugate bases. (b) Evaluate Claim 2 and give the correct order of strength for HF, HCl, HBr and HI. (c) Explain why electronegativity is the deciding factor in Claim 1 but not in Claim 2.
Model answer and suggested Marlbridge rubric
(a) Claim 1 is correct. The conjugate bases are CH₃⁻, NH₂⁻, OH⁻ and F⁻. Across the period the atoms are similar in size, but electronegativity increases from C to F. A more electronegative atom holds a negative charge more securely, so F⁻ is the most stable conjugate base and HF the strongest acid of the four.
(b) Claim 2 is incorrect. The order is HF < HCl < HBr < HI. HF is a weak acid; HCl, HBr and HI are strong acids.
(c) Down a group the atoms change greatly in size. I⁻ is much larger than F⁻, so its charge is spread over a much bigger volume, which stabilises it. The H–I bond is also longer and weaker than the H–F bond, so the proton is released more easily. These size effects outweigh the electronegativity difference. Across a period, size changes little, so electronegativity decides.
| Point | What earns it |
|---|---|
| 1 | Claim 1 supported, linking higher electronegativity to a more stable conjugate base |
| 1 | Claim 2 rejected with the order HF < HCl < HBr < HI (HF weak) |
| 1 | Explains the group trend by anion size (charge spread over a larger volume) or H–X bond strength |
| 1 | Contrasts the two trends: size changes little across a period but a lot down a group |
How did you do?
- Q1 or Q4 wrong: learn the lists of strong acids, strong bases and common weak bases in the study guide.
- Q2 wrong: re-read “Which hydrogens are acidic?”
- Q3 or Q5 wrong: practise naming the feature and its effect on the conjugate base, as in Worked example 1.
- Q6 or Q7 incomplete: your argument needs a claim, the structural evidence and the link to conjugate-base stability.
Then tick off the topic checklist.