Addition of the indicator methyl orange to an unknown solution leads to a yellow color. The addition of bromthymol blue to the same solution also leads to a yellow color. (a) Is the solution acidic, neutral, or basic? (b) Is there another indicator you could use to narrow the range of possible pH values for the solution?

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Identify the pH range where methyl orange changes color. Methyl orange changes from red to yellow in the pH range of approximately 3.1 to 4.4. Since the solution turns yellow, the pH is above 4.4.

Identify the pH range where bromthymol blue changes color. Bromthymol blue changes from yellow to blue in the pH range of approximately 6.0 to 7.6. Since the solution remains yellow, the pH is below 6.0.

Combine the information from both indicators. The solution turns yellow with both indicators, suggesting the pH is between 4.4 and 6.0.

Determine the nature of the solution. Since the pH is between 4.4 and 6.0, the solution is acidic.

Consider another indicator to narrow the pH range. An indicator like phenol red, which changes from yellow to red in the pH range of 6.8 to 8.4, could be used. If the solution remains yellow with phenol red, it confirms the pH is below 6.8, further narrowing the range.

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

pH Scale

The pH scale measures the acidity or basicity of a solution, ranging from 0 to 14. A pH less than 7 indicates an acidic solution, a pH of 7 is neutral, and a pH greater than 7 indicates a basic solution. The color changes of pH indicators, such as methyl orange and bromthymol blue, are directly related to the pH of the solution, providing visual cues about its acidity or basicity.

Indicators

Indicators are substances that change color in response to changes in pH, allowing for the determination of a solution's acidity or basicity. Methyl orange turns yellow in acidic conditions (pH < 3.1) and red in neutral to basic conditions, while bromthymol blue is yellow in acidic solutions (pH < 6.0) and blue in basic solutions (pH > 7.6). The use of multiple indicators can help narrow down the pH range of a solution.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons (H+) between substances, affecting the pH of a solution. In this context, the presence of yellow color from both indicators suggests that the solution is likely acidic, as both indicators indicate low pH. Understanding the behavior of acids and bases, as well as their corresponding indicators, is essential for accurately determining the nature of the unknown solution.

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Acid-Base Reaction

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Textbook Question

The average pH of normal arterial blood is 7.40. At normal body temperature 137 °C2, Kw = 2.4 * 10-14. Calculate 3H+4, 3OH-4, and pOH for blood at this temperature.

Textbook Question

Addition of the indicator methyl orange to an unknown solution leads to a yellow color. The addition of bromthymol blue to the same solution also leads to a yellow color. (b) What is the range (in whole numbers) of possible pH values for the solution?

Textbook Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (b) Which of the following can you establish about the solution: (i) A minimum pH, (ii) A maximum pH, or (iii) A specific range of pH values?

Textbook Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (c) What other indicator or indicators would you want to use to determine the pH of the solution more precisely?