Grade 12 Physical Sciences · Chemical Change · Compare strong and weak acids

Strong and Weak Acids Grade 12

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 · May/June 2025 · Paper 2 · National (DBE) · 2 marks

1.8 The conductivity of two acids,

HNO_{3}

(aq) and

CH_{3} COOH

(aq), each with a concentration of

1

mol·dm

^{- 3}

, is compared at

25

°C. The simplified diagrams below show the apparatus used. The electrodes are made of carbon.

Which ONE of the following combinations CORRECTLY describes the brightness of bulb A and bulb B, and provides the reason for this?(2) Diagram for question 1.8

A A is brighter than B, because $CH_{3} COOH$ is the stronger acid
B A is brighter than B, because $HNO_{3}$ is the stronger acid
C B is brighter than A, because $CH_{3} COOH$ is the weaker acid
D A and B have equal brightness, because the acids are of equal concentration

Work it out on paper first. Then mark yourself with the official memo, one tick at a time.

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 1.8 · 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 (2 ticks)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
5. 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 (2 ticks)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
6. Question 1.7 · November 2023 · 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).
1.7 Nitric acid, $HNO_{3}$ (aq), and ethanoic acid, $CH_{3} COOH$ (aq), of equal volumes and concentrations are compared.

Consider the following statements regarding these solutions:

(i) They have different pH values.
(ii) Both have the same electrical conductivity.
(iii) Both solutions require the same number of moles of KOH(aq) for complete neutralisation.

Which of the above statement(s) is/are TRUE?(2)
1. A (i) only
2. B (i) and (ii) only
3. C (i) and (iii) only
4. D (ii) and (iii) only
Show the memo (2 ticks)Memo
1. ✓
  Correct option(i) $HNO_{3}$ is a strong acid (ionises completely) and $CH_{3} COOH$ is weak (ionises partly), so $HNO_{3}$ has a higher $[H_{3} O^{+}]$ and a lower pH. TRUE.
  (ii) $HNO_{3}$ forms more ions, so it conducts better. FALSE.
  (iii) Both are monoprotic and have the same number of moles of acid, so they need the same number of moles of KOH. TRUE.
  
  Answer: (i) and (iii)
  
  Answer: C
  2 marks · Compare strong and weak acids

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