Indicators and pH scale — KCSE Chemistry

KCSE Chemistry · 55 practice questions · 4 syllabus objectives

21 easy22 medium12 hard

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Explain the pH scale and relate pH values to the concentration of hydrogen ions

Describe the action of natural and synthetic indicators and state their colour changes in acids and bases

Use universal indicator and pH meter to determine the acidity or alkalinity of a solution

Indicators and pH scale

Sample Questions

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1
easySHORT ANSWER4 marks

Identify two indicators that can be used to determine pH and state one property of each. (4 marks)

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Part (a) — 4 marks
Litmus: changes colour from red in acid to blue in alkali (1 mk)
Phenolphthalein: colourless in acid and pink in alkali (1 mk)
Methyl orange: red in acid and yellow in alkali (1 mk)
Bromothymol blue: yellow in acid and blue in alkali (1 mk)
2
easySHORT ANSWER3 marks

Identify the pH range for a weak acid and give an example of such an acid. (3 marks)

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Part (a) — 3 marks
The pH range for a weak acid is typically between 3 and 6 (1 mk)
An example of a weak acid is acetic acid (ethanoic acid) (1 mk)
Another example of a weak acid is citric acid (1 mk)
3
easySHORT ANSWER2 marks

Identify the colour changes observed for bromothymol blue in acidic and alkaline solutions. (2 marks)

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Part (a) — 2 marks
In acidic solutions, bromothymol blue turns yellow (1 mk)
In alkaline solutions, bromothymol blue turns blue (1 mk)
4

State the pH range that indicates a neutral solution and give an example of a neutral substance. (3 marks)

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