Which acid-base indicator is yellow in a solution with a pH of 9.8?

• A. Methyl orange
• B. Bromothymol blue
• C. Phenol red
• D. Phenolphthalein

Answer: (C)

Verified step by step guidance

1. Understand that acid-base indicators change color depending on the pH of the solution, and each indicator has a specific pH range over which it changes color.
2. Recall the typical pH transition ranges for the given indicators: Methyl orange changes color around pH 3.1 to 4.4, Bromothymol blue changes color around pH 6.0 to 7.6, Phenol red changes color around pH 6.8 to 8.4, and Phenolphthalein changes color around pH 8.2 to 10.0.
3. Since the solution has a pH of 9.8, identify which indicator would be in its color transition range or have a stable color at this pH.
4. Note that Phenol red is typically red or pink in basic solutions above pH 8.4, but yellow in acidic to neutral solutions below that range, so at pH 9.8 it would not be yellow.
5. Recognize that Phenolphthalein is colorless in acidic and neutral solutions and turns pink in basic solutions starting around pH 8.2, so it would be pink at pH 9.8, not yellow; therefore, the indicator that is yellow at pH 9.8 is Bromothymol blue, which is yellow in acidic to neutral pH and blue in basic pH, but at pH 9.8 it would be blue, so the correct yellow indicator at pH 9.8 is not among these, indicating a need to carefully check the indicator's color ranges.