Grade 12 Physical Sciences · Chemical Change

Predict salt pH and choose indicator

Predict whether a salt solution is acidic, basic or neutral, and choose a suitable indicator for a titration (sodium ethanoate solution is basic).

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Acids and Bases Grade 12: HydrolysisKevinmathscience

Chemistry | Acids and Bases | Hydrolysis of salts (Simplified)Mlungisi Nkosi Maths & Science

Hydrolysis of Salts - Grade 12Science With Cecile

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Question 7.4 · November 2024 · Paper 2 · National (DBE) · 2 marks

Hydrated potassium carbonate,

K_{2} CO_{3} \cdot x H_{2} O

, is a WEAK BASE. A solution is prepared by dissolving some of this solid in water.

7.4 METHYL ORANGE is used as the indicator. Explain why methyl orange is the most suitable indicator for this titration by referring to the pH at the equivalence point.(2)

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More past paper questions: predict salt ph and choose indicator

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3. Question 7.2.4 · May/June 2025 · Paper 2 · National (DBE) · 2 marks
7.2 The table below shows aqueous solutions of different substances, each of concentration $1$ mol·dm $^{- 3}$ at room temperature.

Identify from the table and write down the FORMULA for:
7.2.4 The solution, which when neutralised with $(COOH)_{2}$ (aq), will have a pH greater than 7(2)
Show the memo (1 tick)Memo
1. ✓
  Strong baseNaOH
  2 marks · Predict salt pH and choose indicator
4. Question 7.1.4 · May/June 2024 · Paper 2 · National (DBE) · 2 marks
7.1 A standard solution is prepared by dissolving 10 g of sodium carbonate, $Na_{2} CO_{3}$ (s), in $0, 7$ dm $^{3}$ of water.

Answer the questions that follow.
The sodium carbonate solution is titrated with dilute hydrochloric acid, HCℓ(aq). The following indicators are available for this titration.
7.1.4 Which ONE of the indicators (P, Q or R) is most suitable for this titration? Give a reason for the answer by referring to the data in the table.(2)
Show the memo (2 ticks)Memo
1. ✓
  IndicatorP
  1 mark · Predict salt pH and choose indicator
2. ✓
  Reason(Titration of) weak base and a strong acid. / The equivalence point is lower than pH 7.
  1 mark · Predict salt pH and choose indicator
5. Question 1.8 · May/June 2023 · Paper 2 · National (DBE) · 2 marks
1.8 Consider the statements below regarding an alkaline substance.

An alkaline substance:

(i) Reacts with an acid to form a neutral solution
(ii) Turns red litmus blue
(iii) Forms a salt when it reacts with an acid

Which of the statements above are ALWAYS TRUE?(2)
1. A (i), (ii) and (iii)
2. B (i) and (ii) only
3. C (i) and (iii) only
4. D (ii) and (iii) only
Show the memo (1 tick)Memo
1. ✓
  Not every neutralisation gives a neutral solution(i) Not always true: the salt formed can hydrolyse. A strong base with a weak acid gives a basic solution; a weak base with a strong acid gives an acidic solution.
  (ii) Always true: a base turns red litmus blue.
  (iii) Always true: acid + base → salt (+ water).
  So only (ii) and (iii).
  
  Answer: D
  2 marks · Predict salt pH and choose indicator
6. Question 1.8 · November 2022 · 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 A dilute acid is titrated against a potassium hydroxide solution, KOH(aq).

At the equivalence point the pH is 7.

Which ONE of the following combinations correctly identifies the acid and the MOST SUITABLE indicator for this titration?(2)
1. A Acid: $(COOH)_{2}$ (aq); indicator: phenolphthalein
2. B Acid: $(COOH)_{2}$ (aq); indicator: bromothymol blue
3. C Acid: $HC ℓ$ (aq); indicator: phenolphthalein
4. D Acid: $HC ℓ$ (aq); indicator: bromothymol blue
Show the memo (1 tick)Memo
1. ✓
  Correct optionpH 7 at the equivalence point means a neutral salt, so the acid must be strong: HCl (KCl does not hydrolyse). Oxalic acid is weak and would give a basic salt (pH > 7). For a strong acid–strong base titration the indicator should change colour near pH 7: bromothymol blue (range about 6–7,6).
  
  Answer: D
  2 marks · Predict salt pH and choose indicator

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