Book cover for Chemistry The Science in Context

Chemistry The Science in Context

Thomas R. Gilbert

ISBN #9780393615142

5th Edition

2,675 Questions

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191,124 Students Helped

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Summary
Learning Objectives
Key Concepts
Example Problems
Explanations
Common Mistakes

Summary

Chapter 11 explores the properties and behavior of solutions with a focus on ionic interactions and colligative properties. It details the influence of factors such as ion charges, interionic distances, and energy changes in the formation and dissolution of ionic compounds. Emphasis is placed on the proper use of concentration units and the van ’t Hoff factor in calculating changes in boiling point, freezing point, and osmotic pressure. Additionally, the chapter connects these concepts to real-world applications, illustrating the practical impact of solution chemistry in various sectors including medicine, engineering, and environmental science.

Learning Objectives

1

Describe the various interactions between ions in ionic solutions and explain how these microscopic interactions influence macroscopic properties.

2

Explain the concept of colligative properties, including boiling point elevation, freezing point depression, and osmotic pressure, and relate them to the number of solute particles.

3

Analyze the energy changes involved in the formation and dissolution of ionic compounds, focusing on ion charges, interionic distances, and associated energy dynamics.

4

Apply concentration units (molality vs. molarity) and incorporate the van ’t Hoff factor to perform accurate colligative property calculations.

5

Examine practical applications of solution behavior, such as fractional distillation, IV fluid preparation, antifreeze formulation, and water desalination by reverse osmosis.

Key Concepts

CONCEPT

DEFINITION

Ionic Interactions

The electrostatic forces between positively and negatively charged ions that determine the behavior and stability of ionic compounds in solution.

Colligative Properties

Properties that depend on the number of solute particles in a solution rather than their identity, including boiling point elevation, freezing point depression, and osmotic pressure.

Molality

A concentration unit defined as the number of moles of solute per kilogram of solvent, crucial for colligative property calculations because it is temperature independent.

Molarity

A concentration unit defined as the number of moles of solute per liter of solution, which can vary with temperature due to volume changes.

van ’t Hoff Factor

A factor used in calculating colligative properties that accounts for the number of particles a solute produces when dissolved.

Interionic Distance

The distance between ions in an ionic compound, which influences the strength of ionic interactions and the energy involved in formation and dissolution.

Example Problems

Example 1

Figure P11.1 shows a particle-level view of a sealed container partially filled with a solution that has two components: $X$ (blue spheres) and $Y$ (red spheres). Which of the following statements about substances $X$ and $Y$ are true? a. $X$ is the solvent in this solution. b. Pure $Y$ is a volatile liquid. c. If $Y$ were not present, there would be fewer X particles in the gas above the liquid solution. d. The presence of $Y$ increases the vapor pressure of $X$.

Example 2

Figure $\mathrm{P} 11.2$ shows a particle-level vicw of a scaled container partially filled with a solution of two miscible liquids: X (blue spheres) and $Y$ (red spheres). Which of the following statements about substances $\mathrm{X}$ and $\mathrm{Y}$ are true? a. Y is the solvent in this solution. b. Pure $Y$ has a higher vapor pressure than pure X. c. The presence of $Y$ in the solution lowers the vapor pressure of $\mathrm{X}$. d. If $Y$ were not present, there would be fewer total particles in the gas above the liquid solution.

Example 3

Figure $P 11.3$ shows particle-level vicws of $0.001 M$ aqueous solutions of the following four solutes: $\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}, \mathrm{NaCl}$ $\mathrm{MgCl}_{2},$ and $\mathrm{K}_{3} \mathrm{PO}_{4} .$ The blue spheres represent particles of solute. a. Which compounds are represented in images (I)-(IV)? b. Which of the four solutions in Figure $P 11.3$ has the highest (i) vapor pressure; (ii) boiling point; (iii) freezing point; (iv) osmotic pressure?

Example 4

The graph in Figure P11.4 describes the volume of distillate collected during the fractional distillation of a liquid. Answer the following questions about the process: (a) Is the sample a pure liquid or a mixture? (b) If it is a mixture: (i) how many components are in the mixture? (ii) What are the relative ratios of the volumes in the mixture? (iii) What are their approximate boiling points?

Example 5

The graph in Figure $P 11.5$ shows the decrease in the freezing point of water $\Delta T_{\mathrm{f}}$ for solutions of two different substances, A (triangles) and B (circles), in water. Explain how you can reasonably conclude that (a) $\mathrm{A}$ and $\mathrm{B}$ are nonelectrolytes and (b) the freezing point depression constant $K_{\mathrm{f}}$ of water is independent of the solute's identity.
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Step-by-Step Explanations

QUESTION

How do you calculate the boiling point elevation of a solution given its molality and the van ’t Hoff factor?

STEP-BY-STEP ANSWER:

Step 1: Identify the molality (m) of the solution and determine the van ’t Hoff factor (i) based on the solute’s dissociation in the solvent.
Step 2: Use the boiling point elevation constant (Kb) for the solvent.
Step 3: Apply the formula ΔTb = i * Kb * m, where ΔTb is the change in boiling point.
Step 4: Add the calculated boiling point elevation (ΔTb) to the normal boiling point of the pure solvent to obtain the new boiling point.
Final Answer: The boiling point of the solution is equal to the pure solvent’s boiling point plus ΔTb as determined by the formula.

Boiling Point Elevation

QUESTION

How can colligative properties be used to determine the molar mass of an unknown solute?

STEP-BY-STEP ANSWER:

Step 1: Prepare a solution with a known mass of solute and a known mass of solvent, and measure a colligative property (e.g., freezing point depression).
Step 2: Calculate the change in freezing point (ΔTf) and use the solvent’s freezing point depression constant (Kf).
Step 3: Use the formula ΔTf = i * Kf * m to solve for molality (m), where i is the van ’t Hoff factor.
Step 4: Convert molality to moles of solute using the mass of the solvent, and then calculate the molar mass by dividing the mass of the solute by the number of moles.
Final Answer: The molar mass of the solute is determined by the ratio of the solute’s mass to the calculated number of moles derived from the observed colligative property.

Measuring Molar Mass using Colligative Properties

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Common Mistakes

  • Confusing molarity with molality, especially in temperature-sensitive applications.
  • Neglecting the effect of the van ’t Hoff factor, leading to miscalculations in colligative properties.
  • Overlooking the significance of interionic distances and energy changes in dissolution and formation processes.
  • Assuming that colligative properties depend on the identity of the solute rather than solely on the number of particles.