Chapter 12, Problem 12.30QP

(a)

Interpretation Introduction

Interpretation:

A solution is made of volatile solute and another solution is made of non-volatile solute.  Of the two solutions the solution having higher vapor pressure and higher boiling point has to be identified.

Concept Introduction:

Vapor pressure of a substance is known as the pressure exerted by molecules on the vapor phase when they are in equilibrium with their actual phase which can be liquid or solid.

A substance is said to be volatile if it vaporizes readily at room temperature itself.  Such substances have high vapor pressure as most of its molecules tend to exist in vapor phase.  A substance is said to be non-volatile if it doesn’t vaporize spontaneously and remains stable.

Vapor pressure of a volatile solvent can be lowered by addition of a non-volatile solute. Raoult’s law deals with the vapor pressure of pure solvents and solution which states –

Partial pressure of solvent is equivalent to the product of vapor pressure of the solvent in its pure state and mole fraction of solvent in the solution.  It is expressed as,

$P_A=P_A^{°}{X}_A$

Where,

$\begin{array}{l}{P}_A\text{ = Partial vapor pressure of solvent in solution}\\ {\text{P}}_A^{°}\text{ = Vapor pressure of pure solvent}\\ {\text{X}}_A\text{= mole fraction of solvent in the solution}\end{array}$

When the solute is non-volatile, the vapor pressure of the whole solution is equal to $P_A.$

The lowering of vapor pressure of the solvent due to the addition of non-volatile solute is expressed as,

$\Delta P=P_A^{°}{X}_B$

Where,

$X_B$ is the mole fraction of the solute.

Boiling point of a liquid substance is defined as the temperature at which the vapor pressure of the liquid becomes equal to the atmospheric pressure.

(b)

Interpretation Introduction

Interpretation:

A solution is made of volatile solute and another solution is made of non-volatile solute.  Of the two solutions the solution having higher vapor pressure and higher boiling point has to be identified.

Concept Introduction:

Vapor pressure of a substance is known as the pressure exerted by molecules on the vapor phase when they are in equilibrium with their actual phase which can be liquid or solid.

A substance is said to be volatile if it vaporizes readily at room temperature itself.  Such substances have high vapor pressure as most of its molecules tend to exist in vapor phase.  A substance is said to be non-volatile if it doesn’t vaporize spontaneously and remains stable.

Vapor pressure of a volatile solvent can be lowered by addition of a non-volatile solute. Raoult’s law deals with the vapor pressure of pure solvents and solution which states –

Partial pressure of solvent is equivalent to the product of vapor pressure of the solvent in its pure state and mole fraction of solvent in the solution.  It is expressed as,

$P_A=P_A^{°}{X}_A$

Where,

$\begin{array}{l}{P}_A\text{ = Partial vapor pressure of solvent in solution}\\ {\text{P}}_A^{°}\text{ = Vapor pressure of pure solvent}\\ {\text{X}}_A\text{= mole fraction of solvent in the solution}\end{array}$

When the solute is non-volatile, the vapor pressure of the whole solution is equal to $P_A.$

The lowering of vapor pressure of the solvent due to the addition of non-volatile solute is expressed as,

$\Delta P=P_A^{°}{X}_B$

Where,

$X_B$ is the mole fraction of the solute.

Boiling point of a liquid substance is defined as the temperature at which the vapor pressure of the liquid becomes equal to the atmospheric pressure.