Problem 39 - Chemistry: The Central Science in SI Units, Global Edition

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. (a) Is the solution acidic, neutral, or basic? (b) What is the range (in whole numbers) of possible pH values for the solution? (c) Is there another indicator you could use to narrow the range of possible pH values for the solution?

Step-by-Step Solution

Answer

(a) The solution is acidic, as its pH range lies between 3.1 and 6.0. (b) The possible pH value range for the solution is 4 to 5. (c) To narrow the pH range further, we can use the indicator "methyl red," which has a transition range between pH 4.4 and 6.2, allowing us to observe color change and further refine the pH range.

Step 1: Research color changes of methyl orange and bromthymol blue

Both methyl orange and bromthymol blue are pH indicators that change color depending on the pH of a solution. Specifically: Methyl orange: - Red in acidic conditions (pH < 3.1) - Yellow in basic conditions (pH > 4.4) - Orange in the transition range (3.1 < pH < 4.4) Bromthymol blue: - Yellow in acidic conditions (pH < 6.0) - Blue in basic conditions (pH > 7.6) - Green in the transition range (6.0 < pH < 7.6)

Step 2: Deduce the pH range of the unknown solution

Given that methyl orange turns yellow in the unknown solution, we can deduce that the solution is either basic or within the transition range of methyl orange (pH > 3.1). Also, bromthymol blue turns yellow in the unknown solution, which means the pH of the solution is less than 6.0 (acidic range of bromthymol blue). Consequently, the possible pH range of the solution lies between 3.1 < pH < 6.0.

Step 3: Determine if the solution is acidic, neutral, or basic

Since the pH range we found in step 2 is greater than 3.1 and less than 6.0: (a) The solution is considered acidic (although it's a weak acid, as the pH is not far below 7).

Step 4: State the range of possible pH values

(b) The range (in whole numbers) of possible pH values for the solution is 4 to 5.

Step 5: Suggest another indicator to narrow the pH range

(c) To narrow the pH range further, we need an indicator with a transition range within our current pH range (3.1 < pH < 6.0). A suitable indicator is "methyl red", which has a transition range between pH 4.4 and 6.2. This indicator will allow us to determine if the solution is closer to pH 4.4 or 6.0 by observing the color change (red) in the acidic pH range and yellow in the transition range (4.4 < pH < 6.2).

Key Concepts

Methyl Orange

Methyl orange is a well-known pH indicator used in laboratory settings to determine the acidity or basicity of a solution. It is particularly handy when you want to discern specific pH ranges. Unlike some other pH indicators, methyl orange displays distinct color changes with slight variations in pH. This makes it easy to interpret when analyzing solutions.

Here's how methyl orange works:

• In acidic conditions, where the pH is less than 3.1, methyl orange turns red. This color indicates a highly acidic environment.
• When the solution is basic, above a pH of 4.4, it transforms to a yellow shade. This yellow color hints that the solution is no longer within the acidic range.
• Between these pH values, specifically from 3.1 to 4.4, methyl orange transitions to an orange color, indicating the mixture contains both acidic and basic properties.

These specific color transitions are why methyl orange is a preferred choice in many chemistry labs. It's very effective in differentiating between low pH values, helping scientists and students alike to gauge the chemical nature of a solution accurately.

Bromthymol Blue

Bromthymol blue is another versatile pH indicator that is often used to identify the pH levels of solutions. Its usefulness comes from its ability to change colors distinctly across a broader pH spectrum than methyl orange, allowing for discerning more varied pH levels.

Bromthymol blue exhibits the following characteristics based on pH levels:

• In highly acidic environments, where the pH is less than 6.0, the indicator presents a yellow color. This suggests that the solution is acidic.
• When the pH is above 7.6, indicating a basic solution, bromthymol blue shows a blue color, leaving no doubt about the basicity of the solution.
• Between the pH levels of 6.0 and 7.6, bromthymol blue shifts to a green hue, marking the transition phase.

This range makes bromthymol blue an ideal choice for identifying whether a substance contains acidic or basic properties, especially when exploring more neutral territories around pH 7.

Acidic Solutions

Acidic solutions are defined by their pH levels, which are less than 7. The lower the pH, the stronger the acidity of the solution. In the context of the exercise you are working on, both methyl orange and bromthymol blue can help in assessing whether a solution is acidic based on their color changes.

To determine whether a solution is acidic, consider the following:

• If methyl orange turns red in your solution, the situation is very acidic (pH < 3.1).
• If bromthymol blue turns yellow, the solution is acidic, having a pH less than 6.0.
• When solutions have pH values between these color change thresholds, the colors can indicate weak acids.

The knowledge of how these indicators behave allows you to classify an unknown solution's acidity precisely. Furthermore, using another indicator with a specific range—such as methyl red (pH transition between 4.4 and 6.2)—can enable you to pinpoint the exact level of acidity. Understanding and applying this information can demystify the process of working with acid solutions effectively.