Grade 11 Physical Sciences · Chemical Change

Compare strong and weak acids

Tell strong from weak acids and concentrated from dilute solutions, and give the reason (HCl ionises fully, so it is a strong acid).

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Grade 12 Acids and Bases Grade 11 Revision PART 2: Strong/Weak vs Concentrated/diluteMiss Martins Maths and Science

GCSE Chemistry - Strong Acids & Weak Acids | pH & Concentration of Hydrogen Ions (2027/28 exams)Cognito

GCSE Chemistry Revision "Strong and Weak Acids"Freesciencelessons

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Question 1.8 · November 2025 · Paper 2 · National (DBE) · 2 marks

Various options are provided as possible answers to the following questions. Each question has only ONE correct answer.

1.8 The concentration of each of the four solutions below is

0, 1

mol·dm

^{- 3}

HNO_{3}

(aq),

NH_{3}

(aq), NaOH(aq) and

H_{2} CO_{3}

(aq)

Which ONE of the following CORRECTLY places the solutions in order of INCREASING pH?(2)

A $H_{2} CO_{3}$ ; $HNO_{3}$ ; $NH_{3}$ ; NaOH
B $HNO_{3}$ ; $H_{2} CO_{3}$ ; $NH_{3}$ ; NaOH
C NaOH ; $NH_{3}$ ; $H_{2} CO_{3}$ ; $HNO_{3}$
D $HNO_{3}$ ; $H_{2} CO_{3}$ ; NaOH ; $NH_{3}$

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More past paper questions: compare strong and weak acids

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3. Question 7.1.1 · November 2025 · Paper 2 · National (DBE) · 2 marks
7.1 Phosphoric acid, $H_{3} PO_{4}$ (aq), is an example of an acid that can ionise in three steps, as shown below.

$H_{3} PO_{4} (aq) + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + H_{2} PO_{4}^{-} (aq)$ $K_{a} = 6, 9 \times 1 0^{- 3}$
$H_{2} PO_{4}^{-} (aq) + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + HPO_{4}^{2 -} (aq)$ $K_{a} = 6, 2 \times 1 0^{- 8}$
$HPO_{4}^{2 -} (aq) + H_{2} O (ℓ) ⇌ H_{3} O^{+} (aq) + PO_{4}^{3 -} (aq)$ $K_{a} = 4, 8 \times 1 0^{- 13}$

Answer the questions that follow.
7.1.1 Which one is the stronger acid, $H_{2} PO_{4}^{-}$ or $HPO_{4}^{2 -}$ ? Give a reason for the answer by referring to the data above.(2)
Show the memo (2 ticks)Memo
1. ✓
  Stronger acid $H_{2} PO_{4}^{-}$
  1 mark · Compare strong and weak acids
2. ✓
  Reason $K_{a}$ of $H_{2} PO_{4}^{-}$ greater/higher than $K_{a}$ of $HPO_{4}^{2 -}$ .
  1 mark · Compare strong and weak acids
4. Question 1.7 · November 2024 · Paper 2 · National (DBE) · 2 marks
Various options are provided as possible answers to the following questions. Each question has only ONE correct answer. Choose the answer and write only the letter (A–D) next to the question numbers (1.1 to 1.10) in the ANSWER BOOK, e.g. 1.11 E.
1.7 The table below shows the ionisation constants, $K_{a}$ , for two acids at $25$ °C.

Consider the following statements for these two acids when they have equal concentration at $25$ °C:

(i) Both are weak acids.
(ii) Butanoic acid is a stronger acid than ethanoic acid.
(iii) The butanoic acid solution has a lower concentration of hydronium ion, $H_{3} O^{+}$ (aq), than the ethanoic acid solution.

Which of the above statements are TRUE?(2)
1. A (i) and (ii) only
2. B (i) and (iii) only
3. C (ii) and (iii) only
4. D (i), (ii) and (iii)
Show the memo (1 tick)Memo
1. ✓
  Correct option(i) Both $K_{a}$ values are much smaller than 1, so both acids are weak. TRUE.
  (ii) Ethanoic acid has the larger $K_{a}$ ( $1, 8 \times 1 0^{- 5}$ against $1, 5 \times 1 0^{- 5}$ ), so ethanoic acid is the stronger acid. FALSE.
  (iii) Butanoic acid ionises less, so at equal concentration it produces fewer $H_{3} O^{+}$ ions. TRUE.
  
  Answer: B
  2 marks · Compare strong and weak acids
5. Question 1.8 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
1.8 Four solutions of different acids of the same concentration are compared.

Which ONE of the following $K_{a}$ values represents the WEAKEST acid at $25$ °C?(2)
1. A $4, 5 \times 1 0^{- 6}$
2. B $2, 5 \times 1 0^{- 5}$
3. C $1, 8 \times 1 0^{- 2}$
4. D $6, 5 \times 1 0^{- 2}$
Show the memo (1 tick)Memo
1. ✓
  Choose the correct optionThe weakest acid ionises least, so it has the SMALLEST $K_{a}$ . Compare the powers of ten first: $1 0^{- 6}$ is smaller than $1 0^{- 5}$ and $1 0^{- 2}$ , so $4, 5 \times 1 0^{- 6}$ is the smallest.
  
  Answer: A
  2 marks · Compare strong and weak acids
6. Question 7.1.2 · May/June 2023 · Paper 2 · National (DBE) · 2 marks
7.1 The conductivity of three acid solutions, A, B and C, as shown below is investigated at the same temperature.

The brightness of the bulb in the apparatus shown below is used as a measure of the conductivity of the solutions.

The acid solutions are electrolytes.

Answer the questions that follow.
The brightness of the bulb for each of the solutions is compared.
7.1.2 In which solution, A or B, will the bulb be brighter? Give a reason for the answer by referring to the types of acids.(2)
Show the memo (2 ticks)Memo
1. ✓
  ChoiceA
  1 mark · Compare strong and weak acids
2. ✓
  Reason $H_{2} SO_{4}$ is diprotic. / Donates more than one mole of $H^{+}$ ions per mole of acid (and both acids are of the same concentration). / $H_{2} SO_{4}$ has a higher $K_{a}$ value.
  1 mark · Compare strong and weak acids

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