   # Vapor-pressure lowering is a colligative property, as are freezing-point depression and boiling-point elevation. What is a colligative property? Why is the freezing point depressed for a solution as compared to the pure solvent? Why is the boiling point elevated for a solution as compared to the pure solvent? Explain how to calculate ∆T for a freezing-point depression problem or a boiling-point elevation problem. Of the solvents listed in Table 10-5, which would have the largest freezing-point depression for a 0.50 molal solution? Which would have the smallest boiling-point elevation for a 0.50 molal solution? A common application of freezing-point depression and boiling-point elevation experiments is to provide a means to calculate the molar mass of a nonvolatile solute. What data are needed to calculate the molar mass of a nonvolatile solute? Explain how you would manipulate these data to calculate the molar mass of the nonvolatile solute. ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243

#### Solutions

Chapter
Section ### Chemistry: An Atoms First Approach

2nd Edition
Steven S. Zumdahl + 1 other
Publisher: Cengage Learning
ISBN: 9781305079243
Chapter 10, Problem 7RQ
Textbook Problem
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## Vapor-pressure lowering is a colligative property, as are freezing-point depression and boiling-point elevation. What is a colligative property? Why is the freezing point depressed for a solution as compared to the pure solvent? Why is the boiling point elevated for a solution as compared to the pure solvent? Explain how to calculate ∆T for a freezing-point depression problem or a boiling-point elevation problem. Of the solvents listed in Table 10-5, which would have the largest freezing-point depression for a 0.50 molal solution? Which would have the smallest boiling-point elevation for a 0.50 molal solution?A common application of freezing-point depression and boiling-point elevation experiments is to provide a means to calculate the molar mass of a nonvolatile solute. What data are needed to calculate the molar mass of a nonvolatile solute? Explain how you would manipulate these data to calculate the molar mass of the nonvolatile solute.

Interpretation Introduction

Interpretation: The colligative properties are to be explained with the molar mass.

Concept Introduction:

Colligative properties of a substance include the depression in the freezing point, elevation of boiling-point and osmotic pressure. These are dependant only on the number present and not based on the solute particles present in an ideal solution. These properties have a direct relationship to the solute particles, and therefore the colligative properties are useful for identifying the nature of solute particles and also calculating the molar masses of substances.

### Explanation of Solution

To define colligative property

Colligative properties include the depression in freezing point, elevation of boiling point and osmotic pressure. Colligative properties are dependant only on the number present and not based on the solute particles present in an ideal solution. These properties have a direct relationship to the solute particles, and therefore the colligative properties are useful for identifying the nature of solute particles and also calculating the molar masses of substances.

To explain why depression in freezing point takes place

When the vapor pressure of the liquid and solid are identical to each other, a substance freezes. The vapor pressure of the liquid gets lowered by addition of solute. In order for the vapor pressure of solution and solid to be identical a lower temperature is required. Hence, when a solution is formed, the freezing point gets depressed.

To explain why elevation in boiling point takes place

When the vapor pressure of the liquid equals the external pressure, a substance boils. A higher temperature is required to reach the point where vapor pressure of the liquid is same as the external pressure because the vapor pressure of liquid is lowered by a solution. Hence, when a solution is formed, the boiling point gets elevated.

To write the formula to calculate the freezing point/boiling point

The depression in freezing point or elevation in boiling point can be given by the equation,

ΔT=Kmsolute

Where, ΔT =change in freezing point dep

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