   Chapter 13.4, Problem 13.10CYU

Chapter
Section
Textbook Problem

Calculate the freezing point of 525 g of water that contains 25.0 g of NaCl. Assume i, the van’t Hoff factor, is 1.85 for NaCl.

Interpretation Introduction

Interpretation: The calculated freezing point of the NaCl solution has to be identified.

Concept introduction:

van’t Hoff factor, i: it is the ration between change in in freezing point measured and change in in freezing point calculated. It indicates the total number of ions that are produced.

van'tHofffactor,i=measuredΔTfpCalculatedΔTfp

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

ΔTfp=Kfpmsolute

where,

Kfp is the molal freezing point depression constant.

Explanation

Given data:

i=1.85Massofwater(solvent)=525g=0.525kgMassofsolute(NaCl)=25.0gKfp(solventwater)  = -1.86oC/m

Molality can be calculated using the equation:

Molality = Moleofsolute(inmol)Amountofsolvent(inKg)

Molality = Moleofsolute(inmol)Amountofsolvent(inkg)=(25.0g/58.4g.mol-1)0.525kg=0.815m

Calculation of freezing point:

ΔTfp measured = Kfp.msolute=(-1.86oC/m)(0

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