   Chapter 13, Problem 55GQ

Chapter
Section
Textbook Problem

Arrange the following aqueous solutions in order of (i) increasing vapor pressure of water and (ii) increasing boiling point. (a) 0.35 m HOCH2CH2OH (a nonvolatile solute) (b) 0.50 m sugar (c) 0.20 m KBr (a strong electrolyte) (d) 0.20 m Na2SO4 (a strong electrolyte)

(a)

Interpretation Introduction

Interpretation: The increasing order of vapour pressure from 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 boiling point
• Osmotic pressure

Boiling point elevation: The boiling point of the solution varies with the solute concentration.

Decline in the boiling 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 boiling point measured and change in in boiling point calculated. It indicates the total number of ions that are produced.

Explanation

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

Vapour pressure of a solution decreases with the increase in concentration of the solute in solution.

• HOCH2CH2OH is a non-volatile solute and hence i=1. So total concentration is 0.35m
• Sugar is a non-electrolyte and hence i=1. So total concentration is 0.50m
• KBr is completely dissolved and produces 2 ions and hence i=2. So total concentration is 0.40m
• Na2SO4 is completely dissolved and produces 3 ions and hence i=3

(b)

Interpretation Introduction

Interpretation: The increasing order of boiling 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 boiling point
• Osmotic pressure

Boiling point elevation: The boiling point of the solution varies with the solute concentration.

Decline in the boiling 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 boiling point measured and change in in boiling point calculated. It indicates the total number of ions that are produced.

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