Chapter 12, Problem 12.36QP

(a)

Interpretation Introduction

Interpretation:

Three beakers of same solution ‘A’, ‘B’ and ‘C’ made of water and non-volatile solute are given –

Figure 1

The solution having higher vapor pressure 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.

(b)

Interpretation Introduction

Interpretation:

Three beakers of same solution ‘A’, ‘B’ and ‘C’ made of water and non-volatile solute are given –

Figure 1

The solution with lowest boiling point has to be identified.

Concept Introduction:

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.

Boiling point of a substance can be determined by the formula, $\Delta T_b{\text{ = iK}}_b\text{m}$.

Where,

$\begin{array}{l}\Delta T_b\text{ = elevation of boiling point}\\ {\text{K}}_b\text{ = ebullioscopic constant}\\ \text{m = molality of the solution; i = Van't Hoff factor}\end{array}$

(c)

Interpretation Introduction

Interpretation:

Three beakers of same solution ‘A’, ‘B’ and ‘C’ made of water and non-volatile solute are given –

Figure 1

A laboratory procedure to make all these three solutions to have same freezing point has to be described.

Concept Introduction:

Freezing point of the substance is temperature at which liquid substance remains in equilibrium with solid substance.