   Chapter 13, Problem 33PS

Chapter
Section
Textbook Problem

A mixture of ethanol C2H5OH, and water has a freezing point of −16.0 °C. (a) What is the molality of the alcohol? (b) What is the weight percent of alcohol in the solution?

(a)

Interpretation Introduction

Interpretation: The molality of the ethanol in a mixture of ethanol and water has to be identified.

Concept introduction:

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

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

Weight percent: The mass of one component divided by the total mass of the mixture, multiplied by 100%

weight % A = Mass of A Mass of A + Mass of B + Mass of C + ...×100%

Explanation

Calculate the molality of ethanol:

Given data:

ΔTfp= -16.0oC-0.0oC=-16.0oCFreezing point of water = 0oCKfp ofWater=-1.86oC/m

Calculation of the molality of solute in solution(m):_Freezing point Depression, ΔTfp= Kfp

(b)

Interpretation Introduction

Interpretation: The weight percentage of the ethanol in a mixture of ethanol and water has to be identified.

Concept introduction:

Weight percent: The mass of one component divided by the total mass of the mixture, multiplied by 100%

weight % A = Mass of A Mass of A + Mass of B + Mass of C + ...×100%

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