   Chapter 13, Problem 49PS

Chapter
Section
Textbook Problem

List the following aqueous solutions in order of increasing melting point. (The last three are all assumed to dissociate completely into ions in water.) (a) 0.1 m sugar (b) 0.1 m NaCl (c) 0.08 m CaCl2 (d) 0.04 m Na2SO4

Interpretation Introduction

Interpretation: The increasing order of melting point from the given aqueous solution 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.

Melting point of a solution increases with the increase in concentration of the solute in solution.

Molality (m): Molality is the number of moles of solute present in one kilogram of solvent.

Molality (m) =Numberofmolesofsolute1kgofsolvent

Explanation

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

Melting point of a solution increases with the increase in concentration of the solute in solution.

• Sugar is a non-electrolyte and hence i=1
• NaCl is completely dissolved and produces 2 ions and hence i=2. So total concentration is 0.2m
• CaCl2 is completely dissolved and produces 3 ions and hence i=3. So total concentration is 0.24m
• Na2SO4 is completely dissolved and produces 3 ions and hence i=3. So total concentration is 0

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