To each of three samples of a solution, a different acid-base indicator is added - VCE - SSCE Chemistry - Question 12 - 2011 - Paper 1
Question 12
To each of three samples of a solution, a different acid-base indicator is added. The following colours are observed.
| Indicator | Colour |
|--------... show full transcript
Worked Solution & Example Answer:To each of three samples of a solution, a different acid-base indicator is added - VCE - SSCE Chemistry - Question 12 - 2011 - Paper 1
Step 1
Determine the pH range for thymol blue
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Answer
Thymol blue changes from red at pH < 1.2 to yellow at pH > 2.8. Therefore, for the indication of yellow, the pH is at least 2.8.
Step 2
Determine the pH range for methyl red
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Answer
Methyl red changes from red at pH < 4.4 to yellow at pH > 6.2. The yellow observed indicates a pH greater than 4.4.
Step 3
Determine the pH range for phenolphthalein
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Answer
Phenolphthalein is colourless in acidic solutions and turns pink at pH 8.2, meaning that in this case it remains colourless, thus indicating that the pH is below 8.2.
Step 4
Combine the pH ranges
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Answer
Based on the individual observations:
pH for thymol blue: >= 2.8
pH for methyl red: > 4.4
pH for phenolphthalein: < 8.2
Combining these constraints indicates that the pH range must be suitable to satisfy all indicators, leading to the conclusion that the pH falls between 4.2 and 8.3.
Step 5
Select the correct option
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Answer
Considering the above analysis, Option D (pH = 8.3 and pH = 10.0) is incorrect, as the pH must be less than 8.3, making Option B (pH = 4.2 and pH = 6.3) the correct answer.
