The graph shows the colour ranges of the acid-base indicators methyl orange, bromothymol blue and phenolphthalein - HSC - SSCE Chemistry - Question 6 - 2001 - Paper 1
Question 6
The graph shows the colour ranges of the acid-base indicators methyl orange, bromothymol blue and phenolphthalein.
Methyl orange
red
yellow
Bromothymol blue
yel... show full transcript
Worked Solution & Example Answer:The graph shows the colour ranges of the acid-base indicators methyl orange, bromothymol blue and phenolphthalein - HSC - SSCE Chemistry - Question 6 - 2001 - Paper 1
Step 1
A solution is yellow in methyl orange
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Answer
Methyl orange turns yellow at a pH range of about 3.1 to 4.4. Therefore, the maximum pH for this indicator must be above 4.4.
Step 2
blue in bromothymol blue
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Answer
Bromothymol blue transitions from yellow to blue at around pH 6.0 to 7.6. Since the solution is blue, the pH must be above 7.6.
Step 3
colourless in phenolphthalein
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Answer
Phenolphthalein is colourless at a pH below 8.2 and turns pink above this value. Therefore, the pH must be less than 8.2.
Step 4
Conclusion
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Answer
Combining all these observations, the pH must be in the range of 7.6 to 8.2. Therefore, the pH range of the solution is: 7.6 < pH < 8.2.
