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Methyl red has the following structure: It undergoes a color change from red to yellow as a solution gets more basic. Calculate an approximate pH range for which methyl red is useful. What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator? What is the color change and the pH at the color change when a weak base is titrated with a strong acid using methyl red as an indicator? For which of these two types of titrations is methyl red a possible indicator?

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Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

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Section
BuyFindarrow_forward

Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 14, Problem 70E
Textbook Problem
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Methyl red has the following structure:

Chapter 14, Problem 70E, Methyl red has the following structure: It undergoes a color change from red to yellow as a solution

It undergoes a color change from red to yellow as a solution gets more basic. Calculate an approximate pH range for which methyl red is useful. What is the color change and the pH at the color change when a weak acid is titrated with a strong base using methyl red as an indicator? What is the color change and the pH at the color change when a weak base is titrated with a strong acid using methyl red as an indicator? For which of these two types of titrations is methyl red a possible indicator?

Interpretation Introduction

Interpretation: The color change of methyl red is given. The color change and the pH at the color change when a weak acid is titrated with a strong base using Methyl red as an indicator; the color change and the pH at the color change when a weak base is titrated with a strong acid using Methyl red as an indicator and the type of indicator for which Methyl red is used as a possible indicator is to be stated.

Concept introduction: The acid-base indicators show the color change with change in pH of the solution.

To determine: The color change and the pH at the color change when a weak acid is titrated with a strong base using Methyl red as an indicator; the color change and the pH at the color change when a weak base is titrated with a strong acid using Methyl red as an indicator and the type of indicator for which Methyl red is used as a possible indicator is to be stated.

Explanation of Solution

Explanation

The pH range for which Methyl red is useful is 4.3-6.3_ .

Given

The value of pKa of Methyl red is 5.0×106 .

An indicator is a weak acid that remains in equilibrium with its conjugate base as,

HIn(aq)+H2O(l)H3O+(aq)+In(aq)

The pH change for the above reaction is given as,

pH=pKa+log[In][HIn] (1)

The indicators show a change in color in pKa range of ±1 . It means that the solution will retain their color till the time there is presence of at least 10% amount of other species. The value of pH=pKa+1 when the concentration of salt is ten times greater than acid while pH=pKa1 when the concentration of acid is ten times greater than the salt.

The relationship between pKa and Ka is given as,

pKa=logKa

Where,

  • Ka is the dissociation constant of an acid.

Substitute the value of Ka in the above equation.

pKa=logKa=log(5.0×106)=5.30

Therefore, using the relation pH=pKa±1 , the value of pH comes out to be 4.3-6.3_ .

The pH range for which Methyl red is useful is 4.3-6.3_ .

Explanation

The pH of the solution when a weak acid is titrated with a strong base is 6.3_ and the color change is from red to yellow.

The dissociation of any indicator is shown as,

HIn(aq)+H2O(l)H3O+(aq)+In(aq)

The value of. Ka is given as,

Ka=[H3O+][In][HIn]

When a weak acid is titrated with strong base, the solution will be more basic therefore, the ratio of [In][HIn] is equal to 101

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Chemistry: An Atoms First Approach
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