Chapter 16.3, Problem 32LC

(a)

Interpretation Introduction

Interpretation:

Between $\text{KI}$ and $\text{MgI2}$ , which one has the higher boiling point is to be explained.

Concept Introduction:

The boiling point is the temperature at which the substance goes from the liquid state to the vapor state. This process occurs when the atmospheric pressure and the vapor pressure of the liquid become equal.

Answer to Problem 32LC

  $\text{MgI2}$ will have a higher boiling point than $\text{KI}$ .

Explanation of Solution

The formula for boiling point is as follows:

  $\Delta Tb=i\cdot Kb\cdot m$

Where:

  $\Delta Tb$ is the elevation in boiling point.

  $i$ is Van’t Hoff factor.

  $Kb$ is the ebullioscopic constant.

  $m$ is the molality.

The more is the value of $\Delta Tb$ , the higher will be the boiling point, and vice versa. The value of $i$ is the deciding factor here.

The ionization of both $\text{KI}$ and $\text{MgI2}$ is as follows:

  $\text{KI }\to {\text{ K}}^{+}{\text{ + I}}^{-}$

  $\text{MgI2 }\to {\text{ Mg}}^{2+}{\text{ + 2I}}^{-}$

After ionization, $\text{KI}$ gives $2$ ions and $\text{MgI2}$ gives $3$ ions which means the value of $i$ is $2$ for $\text{KI}$ and $3$ for $\text{MgI2}$ . As more ions are provided by $\text{MgI2}$ hence, the boiling point of $\text{MgI2}$ will be higher because more kinetic energy needs to be given to the $\text{MgI2}$ solution than $\text{KI}$ solution. This applies when an equal number of moles of $\text{KI}$ and $\text{MgI2}$ are dissolved in an equal volume of water.

(b)

Interpretation Introduction

Interpretation:

Between $\text{KI}$ and $\text{MgI2}$ , which one has the higher vapor pressure is to be explained.

Concept Introduction: Vapor pressure is the pressure exerted by the vapors on its liquid in a closed system.

Answer to Problem 32LC

  $\text{KI}$ will have higher vapor pressure than $\text{MgI2}$ .

Explanation of Solution

  $\text{MgI2}$ have a higher boiling point which means more kinetic energy has to be provided to this solution in order to increase its vapor pressure so, that the solution can boil. That means the solution has a lower vapor pressure which is why the boiling point is more. Therefore, $\text{KI}$ will have higher vapor pressure as it boils earlier than $\text{MgI2}$ .

(c)

Interpretation Introduction

Interpretation:

Between $\text{KI}$ and $\text{MgI2}$ , which one has the higher freezing point is to be explained.

Concept Introduction: The freezing point is the temperature at which the substance goes from liquid state to solid state. Basically, at this temperature, the substance solidifies.

Answer to Problem 32LC

  $\text{KI}$ has a higher freezing point than $\text{MgI2}$ .

Explanation of Solution

Freezing point is defined by the formula below:

  $\Delta Tf=i\cdot Kf\cdot m$

Where:

  $\Delta Tf$ is the depression in freezing point.

  $i$ is the Van’t hoff factor.

  $Kf$ is the cryoscopic constant.

  $m$ is the molality of the solution.

The higher the value of $\Delta Tf$ , the lower will be the freezing point, and vice versa.

The value of $i$ will be the deciding factor here as, the value of $i$ for $\text{KI}$ is $2$ whereas, the value of $i$ for $\text{MgI2}$ is $3$ . Therefore, the value of $\Delta Tf$ is more in case of $\text{MgI2}$ and hence, $\text{MgI2}$ will have a lower freezing point and $\text{KI}$ will have a higher freezing point.