   Chapter 13, Problem 50PS

Chapter
Section
Textbook Problem

Arrange the following aqueous solutions in order of decreasing freezing point. (The last three are all assumed to dissociate completely into ions in water.) (a) 0.20 m ethylene glycol (nonvolatile, nonelectrolyte) (b) 0.12 m K2SO4 (c) 0.10 m MgCl2 (d) 0.12 m KBr

Interpretation Introduction

Interpretation: The decreasing order of freezing point for the given aqueous solutions has to be determined.

Concept introduction:

Colligative properties: Properties of solutions which having influence on the concentration of the solute in it. Colligative properties are,

• Decrease in the vapor pressure
• Increase in the boiling point
• Decline in the freezing point
• Osmotic pressure

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

Freezing point depression = ΔTfp= Kfp. msolute.i,where,Kfp=molal freezing point depression constant,msolute= molality of solute,i=van'tHofffactor.

Decline in the freezing point is huge when solute is an electrolyte than when solute is nonelectrolyte.

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.

Freezing point of a solution decreases with the increase in concentration of the solute in solution.

Explanation

Colligative properties are the properties of solutions which having influence on the concentration of the solute in it.

Freezing point of a solution decreases with the increase in concentration of the solute in solution.

• Ethylene glycol is a non-electrolyte and hence i=1. So total concentration is 0.20m
• K2SO4 is completely dissolved and produces 3 ions and hence i=3. So total concentration is 0.36m
• MgCl2 is completely dissolved and produces 3 ions and hence i=3. So total concentration is 0.30m
• KBr is completely dissolved and produces 2 ions and hence i=2

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