Solutions and Raoult's Law

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Questions and Answers

What characterizes a nonideal solution with positive deviation from Raoult's law?

It has higher vapour pressure than the pure components. (correct)

The intermolecular attractions are stronger than solvent-solvent interactions.

It has lower vapour pressure than the pure components.

It forms a solid phase at lower temperatures.

In a solution with negative deviation from Raoult's law, what type of intermolecular interactions are present?

Solvent-solvent interactions are stronger.

Solute-solute interactions are stronger.

Solute-solvent interactions are stronger. (correct)

Interactions are equal across all types.

What is the relationship stated in Raoult's law for a solution of a nonvolatile solute?

The vapour pressure is equal to the pure solvent's vapour pressure multiplied by the mole fraction of the solvent. (correct)

The vapour pressure is independent of solution concentration.

The vapour pressure of the solution is the same as that of the pure solute.

The vapour pressure increases with an increase in solute concentration.

What is indicated by the term ΔP in Raoult's law?

The lowering of vapour pressure. Signup and view all the answers

How does temperature affect the vapour pressure of a liquid?

Vapour pressure increases with higher temperature. Signup and view all the answers

What does the mole fraction of the solute indicate in the context of vapour pressure lowering?

It is directly proportional to the decrease in vapour pressure. Signup and view all the answers

Which example illustrates a positive deviation from Raoult's law?

Acetone and ethanol. Signup and view all the answers

Which statement about Raoult's law is true for solutions of nonvolatile solutes?

Mole fractions must sum to one for solute and solvent. Signup and view all the answers

What defines the normal boiling point of a liquid?

It is the temperature where vapour pressure equals the external pressure. Signup and view all the answers

What indicates that a solution has an abnormal colligative property?

The observed colligative property is higher or lower than the theoretical value. Signup and view all the answers

What happens to colligative properties when a solute dissociates in a polar solvent?

The number of solute particles increases. Signup and view all the answers

What occurs when a nonelectrolyte solute aggregates in a nonpolar solvent?

The number of particles in solution decreases. Signup and view all the answers

When KCl is dissolved in water, what is the result of its dissociation?

It increases the number of solute particles in the solution. Signup and view all the answers

In which scenario is the observed colligative property lower than expected?

When the solute forms fewer particles through association. Signup and view all the answers

What is the effect of temperature on vapour pressure?

It causes an increase in vapour pressure. Signup and view all the answers

What characterizes a colligative property?

It is influenced by the number of solute particles in the solution. Signup and view all the answers

Study Notes

Solutions and Raoult's Law

Positive Deviations from Raoult's Law: Solutions with higher vapor pressures than predicted by Raoult's Law exhibit positive deviations. Solute and solvent interactions are weaker than the interactions within the pure components. Examples include acetone and ethanol, and carbon disulfide and acetone.
Negative Deviations from Raoult's Law: Solutions with lower vapor pressures than predicted by Raoult's Law exhibit negative deviations. Solute and solvent interactions are stronger than the interactions within the pure components. Examples include phenol and aniline, and chloroform and acetone.

Raoult's Law for Nonvolatile Solutes

Statement: The vapor pressure of a solvent over a solution of a nonvolatile solute is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the solvent at constant temperature.
Mathematical Representation: P = x₁P₀, where P is the vapor pressure of the solution, x₁ is the mole fraction of the solvent, and P₀ is the vapor pressure of the pure solvent.
Relative Lowering of Vapor Pressure: The difference in vapor pressure between the pure solvent (P₀) and the solution (P) is proportional to the mole fraction of the solute (x₂): ΔP = x₂P₀.

Temperature and Vapor Pressure

Vapor pressure increases with temperature as more molecules gain enough energy to escape the liquid phase and enter the gas phase.
The normal boiling point is the temperature at which the vapor pressure equals the external pressure (typically atmospheric pressure).

Abnormal Colligative Properties

Colligative properties depend on the number of solute particles in the solution, not their nature.
Abnormal values arise when the solute dissociates or associates in the solution, changing the number of particles.
Dissociation: Electrolytes like KCl dissociate into ions (K⁺ and Cl⁻) in polar solvents, increasing the number of particles and resulting in higher observed colligative properties.
Association: Nonelectrolytes in nonpolar solvents can associate, forming larger molecules and decreasing the number of particles. This leads to lower observed colligative properties. Examples include acetic acid (2CH₃COOH → (CH₃COOH)₂) and benzoic acid (2C₆H₅COOH → (C₆H₅COOH)₂).

Degree of Dissociation and Molar Mass

The degree of dissociation (α) is the fraction of solute molecules that dissociate into ions.
The van't Hoff factor (i) is the ratio of the observed colligative property to the theoretical value based on the undissociated solute.
The relationship between α, i, and the molar mass (M) is: i = 1 + (α-1)n, where n is the number of ions formed per molecule of solute.

van't Hoff Factor

The van't Hoff factor (i) quantifies the deviation from ideal colligative behavior.
i = 1 for non-dissociating solutes.
i > 1 for dissociating solutes, indicating an increase in the number of particles.
i < 1 for associating solutes, indicating a decrease in the number of particles.

Elevation of Boiling Point and Depression of Freezing Point

Elevation of Boiling Point (ΔTb): The boiling point of a solution is higher than that of the pure solvent.
Depression of Freezing Point (ΔTf): The freezing point of a solution is lower than that of the pure solvent.
Mathematical Representations: ΔTb = Kbm and ΔTf = Kfm, where Kb and Kf are the molal elevation constant and molal depression constant, respectively, and m is the molality of the solution.
Graph: A plot of ΔTb or ΔTf against molality results in a straight line passing through the origin.
Units: ΔTb and ΔTf are typically measured in degrees Celsius (°C).

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Description

This quiz focuses on the concepts of Raoult's Law, including positive and negative deviations and their implications in solutions. You will explore examples of solutions that demonstrate these deviations, as well as the mathematical representation of vapor pressure in nonvolatile solute solutions.