Chapter 16, Problem 16.10VP

(a)

Interpretation Introduction

Interpretation: The Figures corresponding to the answers to the given questions are to be identified.

Concept introduction: At low $\text{pH}$, a weak acid remains undissociated, whereas, at high $\text{pH}$, the weak acid dissociates.

When a strong acid is added to the solution of weak acid and its conjugate base, the protons of the acid reacts with the conjugate base to form the undissociated acid and when a strong base is added to the solution of a weak acid and its conjugate base, the hydroxide ion of the base reacts with the undissociated acid to form a more amount of conjugate base.

To determine: The Figure that represents the molecular formula of methyl red at high and low $\text{pH}$ from the Figure $\text{C}$ and $\text{G}$.

Answer to Problem 16.10VP

Solution:

Figure $\text{C}$ represents the molecular formula of methyl red at low $\text{pH}$ and Figure $\text{G}$ represents the molecular formula of methyl red at high $\text{pH}$.

Explanation of Solution

Carboxylic group in methyl red behave as acidic group. At low $\text{pH}$, that is when the solution is acidic, the carboxylic group does not dissociate to give protons being a weak acid.

Therefore, the Figure that represents the molecular structure of methyl red at low $\text{pH}$ is Figure $\text{C}$.

Figure 1

At high $\text{pH}$, that is when the solution is basic, the carboxylic group dissociates to give protons being a weak acid.

Therefore, the Figure that represents the molecular structure of methyl red at high $\text{pH}$ is Figure $\text{G}$.

Figure 2

(b)

Interpretation Introduction

To determine: The Figure that represents the color of methyl red at low and high $\text{pH}$ from the Figure $\text{A}$ and $\text{I}$.

Answer to Problem 16.10VP

Solution:

Figure $\text{A}$ represents the color of methyl red at low $\text{pH}$ and Figure $\text{I}$ represents the color of methyl red at high $\text{pH}$.

Explanation of Solution

Carboxylic group in methyl red behave as acidic group. At low $\text{pH}$, that is when the solution is acidic, the carboxylic group does not dissociate to give protons being a weak acid.

The color of methyl red in acidic solution is red.

Therefore, the Figure that represents the color of methyl red at low $\text{pH}$ is Figure $\text{A}$.

Figure 3

At high $\text{pH}$, that is when the solution is basic, the carboxylic group dissociates to give protons being a weak acid.

The color of methyl red in basic solution is yellow.

Therefore, the Figure that represents the color of methyl red at high $\text{pH}$ is Figure $\text{I}$.

Figure 4

(c)

Interpretation Introduction

To determine: The Figure that represents the buffer solution containing the equal concentration of the weak acid and its conjugate base.

Answer to Problem 16.10VP

Solution:

Figure $\text{B}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base.

Explanation of Solution

The space filling molecular models of the yellow sphere and yellow spheres attached with the white sphere is given.

Therefore, the two spheres attached represent a weak acid and the single yellow sphere represents its conjugate base.

In Figure $\text{B}$, the number of single yellow spheres is equal to that of the two spheres attached.

Therefore, Figure $\text{B}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base.

Figure 5

(d)

Interpretation Introduction

To determine: The Figure that represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added.

Answer to Problem 16.10VP

Solution:

Figure $\text{E}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added and Figure $\text{H}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added.

Explanation of Solution

The space filling molecular models of the yellow sphere and yellow spheres attached with the white sphere is given.

Therefore, the two spheres attached represent a weak acid and the single yellow sphere represents its conjugate base.

When a strong acid is added to the solution of weak acid and its conjugate base, the protons of the acid reacts with the conjugate base to form the undissociated acid and when a strong base is added to the solution of a weak acid and its conjugate base, the hydroxide ion of the base reacts with the undissociated acid to form a more amount of conjugate base.

Therefore, Figure $\text{E}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added.

Figure 6

Figure $\text{H}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added.

Figure 7

(e)

Interpretation Introduction

To determine: The Figure that represents the saturated solution of calcium sulphide and calcium fluoride from the Figure $\text{D}$ and $\text{F}$.

Answer to Problem 16.10VP

Solution:

Figure $\text{D}$ represents the saturated solution of calcium sulphide and Figure $\text{F}$ represents the saturated solution of calcium fluoride.

Explanation of Solution

The molecular formula of calcium sulphide and calcium fluoride is $\text{CaS}$ and ${\text{CaF}}_2$, respectively.

Therefore, calcium sulphide on dissolution dissociates into one calcium ion and one sulphide ion, whereas, calcium fluoride on dissolution dissociates into one calcium ion and two fluoride ion.

Figure $\text{D}$ represents the solution containing two ions, whereas, Figure $\text{F}$ represents the solution containing three ions.

Therefore, the Figure that represents the saturated solution of calcium sulphide is Figure $\text{D}$.

Figure 8

The Figure $\text{F}$ represents the saturated solution of calcium fluoride is Figure $\text{F}$.

Figure 9

(f)

Interpretation Introduction

To determine: The change in solubility of calcium sulphide and calcium fluoride on addition of nitric acid to the saturated solution.

Answer to Problem 16.10VP

Solution:

The solubility of calcium sulphide highly increases, whereas, the solubility of calcium fluoride slightly increases on addition of nitric acid to the saturated solution of calcium sulphide and calcium fluoride, respectively.

Explanation of Solution

Calcium sulphide reacts with nitric acid to from calcium nitrate and hydrogen sulphide.

Therefore, the addition of the nitric acid to the saturated solution of calcium sulphide causes increase in the solubility of calcium sulphide to increase.

Calcium fluoride does not react much with nitric acid. Besides this, the solubility of calcium fluoride slightly increases in acidic medium.

Conclusion:

1. a. Figure $\text{C}$ represents the molecular formula of methyl red at low $\text{pH}$ and Figure $\text{G}$ represents the molecular formula of methyl red at high $\text{pH}$.
2. b. Figure $\text{A}$ represents the color of methyl red at low $\text{pH}$ and Figure $\text{I}$ represents the color of methyl red at high $\text{pH}$.
3. c. Figure $\text{B}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base.
4. d. Figure $\text{E}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added and Figure $\text{H}$ represents the buffer solution containing the equal concentration of the weak acid and its conjugate base to which a strong acid a strong base is added.
5. e. Figure $\text{D}$ represents the saturated solution of calcium sulphide and Figure $\text{F}$ represents the saturated solution of calcium fluoride.
6. f. The solubility of calcium sulphide highly increases, whereas, the solubility of calcium fluoride slightly increases on addition of nitric acid to the saturated solution of calcium sulphide and calcium fluoride, respectively