Chapter U6, Problem 10RE

(a)

Interpretation Introduction

Interpretation: The reaction for the dissociation of the $\text{HIn}$ molecule in solution needs to be determined.

Concept Introduction: In a chemical reaction, the reactants are written on the left side of the equation and the product is written on the right side. The product and the reactants contain an arrow between them showing the proceeding of the reaction from the reactants to the products.

Answer to Problem 10RE

The reaction for the dissociation of the $\text{HIn}$ molecule in solution is,

  $\text{HIn}\left(\text{aq}\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(\text{aq}\right)\text{+}{\text{In}}^{\text{-}}\left(\text{aq}\right)$

Explanation of Solution

Given, for the acid-base indicator litmus, the molecule $\text{HIn}\left(\text{aq}\right)$ is red and the anion, ${\text{In}}^{\text{-}}\left(\text{aq}\right)$ is blue.

The reaction for the dissociation of the $\text{HIn}$ molecule in solution is,

  $\text{HIn}\left(\text{aq}\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(\text{aq}\right)\text{+}{\text{In}}^{\text{-}}\left(\text{aq}\right)$

(b)

Interpretation Introduction

Interpretation: Why is litmus purple when dissolved in water needs to be explained.

Concept Introduction: An indicator is a substance that changes its original color in acidic and basic medium, such as Litmus.

Answer to Problem 10RE

The combination of red and blue color gives rise to the purple color of the litmus when dissolved in water which is due to the presence of both $\text{HIn}$ and ${\text{In}}^{-}$ present in the solution.

Explanation of Solution

When the acid-base indicator is dissolved in water, it exists in $\text{HIn}$ form and also in the form of its ions. Hence, both $\text{HIn}$ and ${\text{In}}^{-}$ are present in the solution. Therefore, the combination of red and blue color gives rise to the purple color of the litmus when dissolved in water.

(c)

Interpretation Introduction

Interpretation: What color do litmus show in a basic solution using Le Chatelier’s needs to determined.

Concept Introduction: An indicator is a substance that changes its original color in acidic and basic medium, such as Litmus.

Answer to Problem 10RE

The litmus show blue color in a basic solution.

Explanation of Solution

In the reaction,

  $\text{HIn}\left(\text{aq}\right)\rightleftharpoons {\text{H}}^{\text{+}}\left(\text{aq}\right)\text{+}{\text{In}}^{-}\left(\text{aq}\right)$

When a base is added to the solution, then less ${\text{H}}^{\text{+}}$ is available in the solution. It makes the reaction faster and generates more ${\text{H}}^{\text{+}}$ and ${\text{In}}^{-}$ . Therefore, the color will change to blue.