Ch.13 - Properties of Solutions

Problem 9b

Chapter 13, Problem 9b

The figure shows two identical volumetric flasks containing the same solution at two temperatures. (b) Does the molality of the solution change with the change in temperature? [Section 13.4]

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Understand the concept of molality: Molality (m) is defined as the number of moles of solute per kilogram of solvent. It is expressed as \( m = \frac{\text{moles of solute}}{\text{kilograms of solvent}} \).

Recognize that molality is independent of temperature: Unlike molarity, which depends on the volume of the solution and can change with temperature, molality is based on the mass of the solvent, which does not change with temperature.

Consider the effect of temperature on the solution: When temperature changes, the volume of the solution may expand or contract, but the mass of the solvent remains constant.

Conclude that since molality is based on the mass of the solvent and not the volume of the solution, it does not change with temperature.

Therefore, the molality of the solution remains constant despite changes in temperature.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Molality

Molality is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. Unlike molarity, which is affected by changes in volume due to temperature, molality remains constant because it is based on the mass of the solvent, which does not change with temperature.

Effect of Temperature on Solutions

Temperature can influence the physical properties of a solution, such as density and viscosity, but it does not affect the molality. As temperature increases, the kinetic energy of the molecules increases, which may lead to changes in solubility, but the mass of the solvent remains unchanged, keeping the molality constant.

Identical Volumetric Flasks

Using identical volumetric flasks ensures that the volume of the solution remains constant regardless of temperature changes. This consistency is crucial for comparing the properties of the solutions at different temperatures, as it eliminates volume as a variable, allowing for a clear focus on how temperature affects other properties like molality.

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If you compare the solubilities of the noble gases in water, you find that solubility increases from smallest atomic weight to largest, specifically: Ar < Kr < Xe. Which of the following statements is the best explanation? [Section 13.3] (a) The heavier the gas, the more it sinks to the bottom of the water and leaves room for more gas molecules at the top of the water. (b) The heavier the gas, the more dispersion forces it has, and therefore the more attractive interactions it has with water molecules. (c) The heavier the gas, the more likely it is to hydrogen-bond with water. (d) The heavier the gas, the more likely it is to make a saturated solution in water.

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The structures of vitamins E and B6 are shown below. Predict which is more water soluble and which is more fat soluble. [Section 13.3]

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You take a sample of water that is at room temperature and in contact with air and put it under a vacuum. Right away, you see bubbles leave the water, but after a little while, the bubbles stop. As you keep applying the vacuum, more bubbles appear. A friend tells you that the first bubbles were water vapor, and that the low pressure had reduced the boiling point of water, causing the water to boil. Another friend tells you that the first bubbles were gas molecules from the air (oxygen, nitrogen, and so forth) that were dissolved in the water. Which friend is most likely to be correct? What, then, is responsible for the second batch of bubbles? [Section 13.4]

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Textbook Question

Suppose you had a balloon made of some highly flexible semipermeable membrane. The balloon is filled completely with a 0.2 M solution of some solute and is submerged in a 0.1 M solution of the same solute:

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