Chapter 5, Problem 5F.2E

Interpretation Introduction

Interpretation:

When $\text{1}\text{.78}\text{g}$ of nonpolar solute dissolved in $\text{60}\text{.0}\text{g}$ of phenol, the freezing point of phenol is lowered by $\text{1}{\text{.362}}^{\text{°}}\text{C}$.  The molar mass of solute has to be calculated.

Concept Introduction:

By using the Cryoscopy method, the molar mass of solute can be calculated as follows:

Step-1: The observed freezing point depression, ${\text{ΔT}}_{\text{f}}$ has to be converted into solute molality in the form of

  $\text{b}\text{=}\frac{{\text{ΔT}}_{\text{f}}}{\text{i}{\text{k}}_{\text{f}}}$

Step-2: The amount of solute (in moles) in the sample is calculated by multiplying the molality by the mass of solvent (in kilograms).

  ${\text{n}}_{\text{solute}}\text{=}\text{b}\text{×}{\text{m}}_{\text{solvent}}$

Step-3: The molar mass of the solute is calculated by dividing the given mass of solute (in grams) by the calculated amount in moles.

  ${\text{M}}_{\text{solute}}\text{=}\frac{{\text{m}}_{\text{solute}}}{{\text{n}}_{\text{solute}}}$