Question

Suppose you are given an unknown solution and you need to test its pH. The pH meter is broken, and all you have is the indicator bromothymol blue to use. You add two drops of bromothymol blue to 10 mL of your unknown, and the solution turns a bright yellow. Which of the following is the best estimate of the pH of the unknown solution?

a. 8.5
b. 11.5
c. 1.5
d. 7.0

Answer 1

Answer: The best estimate of the pH of the unknown solution is 1.5. Option C.

Step-by-step explanation:

A substance which shows different colours in acidic and basic media is called an indicator. Typical examples of common indicators includes:

--> Litmus: colour in acid ( red), colour in base (blue) and colour in neutral (purple).

--> Methyl orange: Colour in acid (red), colour in base (yellow) and colour in neutral (Orange).

--> Phenolphthalein: Colour in acid (colourless), colour in base (pink) and colour in neutral (colourless).

--> BROMOTHYMOL BLUE: Colour in acid ( yellow), colour in base (blue) and colour in neutral (green).

The indicators are themselves weak organic acids or bases and hence slightly dissociated in solution. The colour of the indicator at any given point depends on the relative proportion of the dissociated ions to the undissociated molecules since they have different colours.

From the question, since the pH meter is broken, and the pH of the unknown solution is to be determined,using an indicator instead. The best estimate of the pH of the unknown solution is 1.5. This is so because BROMOTHYMOL BLUE is blue when the pH is greater than 7.6, green when the pH is between 6-7.6, and yellow when the pH is less than 6. And pH value of 1.5 is less than 6, indicating the solution is very acidic.