   Chapter 13.4, Problem 13.8CYU

Chapter
Section
Textbook Problem

An aluminum-containing compound has the empirical formula (C2HS)2AIF. Find the molecular formula if 0.448 g of the compound dissolved in 23.46 g of benzene has a freezing point of 5.265 °C.

Interpretation Introduction

Interpretation: The molecular formula of the given aluminum compound to be identified.

Concept introduction:

Freezing point depression: The freezing point of the solution varies with the solute concentration.

Freezing point depression = ΔTfp= Kfp. msolute,where,Kfpismolal freezing point depression constant,msoluteis molality of solute

Explanation

Given data:

Mass of solute = 0.448 g.Mass of Benzene  = 23.46 g = 0.0235 kgEmpirical formula is (C2H5)2AlF and the mass is104 g/molFreezing point=5.265oC

Freezing point depression = ΔTfp5.265oC - 5.50oC = - 0.235oC

Using the equation of freezing point depression, mass of the solute has to be calculated.

Known data:

Kfp (benzene) = - 5.12oC/m

Freezing point depression = ΔTfp= Kfp. msolute,msoluteΔTfpKfp =- 0.235oC- 5.12oC/m=0.046m

From the equation of molality, mass of solute can be determined.

Moles of solute (mol)  = (molality) (Amount of solvent(in kg))                                    = (0

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