Chapter 14, Problem 71E

Interpretation Introduction

(a)

Interpretation:

The value of K_f for cyclohexane should be determined.

Concept introduction:

Freezing point depression of a solvent occurs due to the addition of nonvolatile solute. The amount that the freezing point is lowered is proportional to the mole fraction of the solute. In dilute solutions, solute mole fraction is equal to its molality. The freezing point depression can be determined by

$\Delta T_f=-K_f\times m$

$\Delta T_f$ - freezing point depression (⁰C)

$K_f$ - proportionality constant ( ⁰C mol-1 kg) (depends on melting point, enthalpy of fusion, molar mass of the solvent)

m − molality (mol/kg)

Interpretation Introduction

(b)

Interpretation:

The better solvent for molar mass determinations by freezing point depression should be determined.

Concept introduction:

The freezing point depression can be used to determine molar mass of an unknown compound. The freezing point depression can be determined by

$\Delta T_f=-K_f\times m$

$\Delta T_f$ - freezing point depression (⁰C)

$K_f$ - proportionality constant (⁰C mol-1 kg) (depends on melting point, enthalpy of fusion, molar mass of the solvent)

m − molality (mol/kg)

K_f indicates the extent of the depression of freezing point per 1 molal solution.