13.3.1 Vapor Pressure Lowering Flashcards Preview

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Flashcards in 13.3.1 Vapor Pressure Lowering Deck (12)
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1
Q

Vapor Pressure Lowering

A
  • A colligative property is one that depends on the concentration of the solute but not on the nature of the solute.
  • Raoult’s law states that the vapor pressure of the solvent decreases linearly as the mole fraction of the solute increases.
2
Q

note

A
  • Raoult’s law states that as solute is added to a solution the vapor pressure of the solvent decreases.
  • In an ideal solution, solute molecules are completely free to move around and disperse, which increases the entropy of the system. If molecules of the solvent move into the gas phase, the volume of the solvent is reduced, and therefore the entropy of the solution decreases. This decrease in entropy stabilizes the liquid phase. Vapor pressure decreases as a result.
  • Using Raoult’s law, a change in the vapor pressure of 78.0 g of benzene after 6.40 g of naphthalene is dissolved can be calculated from the mole fraction of the solvent and its original vapor pressure (0.1252 atm).
  • First, the moles of benzene and moles of naphthalene must be calculated in order to obtain the mole fraction of benzene.
  • The mole fraction of the solvent and its original vapor
    pressure are then plugged into Raoult’s law to give the new vapor pressure.
  • When there are significant attractions between solute and solvent, ΔH is negative (exothermic), and the liquid phase is further stabilized. This means more energy is required to go from liquid to vapor. The vapor pressure will be lower than that expected for an ideal solution.
  • If the addition of solute causes more bonds between solvent molecules to be disrupted, then ΔH will be positive (endothermic). The formation of solution costs energy, and therefore less energy is required to convert solvent molecules from liquid to vapor. The vapor pressure will be higher than that expected for an ideal solution.
3
Q

Which of the following is not an example of a colligative property?

A

solute-concentration elevation

4
Q

Look at the plot and the equations for Raoult’s law. Which statement is not correct with respect to Raoult’s law?

A

If you increase the concentration of the solvent, Xsolvent, the vapor pressure of the solute, Psolute, increases.

5
Q

What is Raoult’s law?

A

Raoult’s law states that the vapor pressure of the solvent in a solution is directly proportional to the mole fraction of the solvent.

6
Q

A simple experiment proves Raoult’s law that the presence of solute molecules in a solution lowers the vapor pressure. Look at the diagram representing a closed system.

You can see that, after time, the pure water in the closed system evaporated and condensed into the sugar water. Which statement about this experiment is not true?

A

The pure water has a greater vapor pressure. Water molecules will evaporate from the solution that has less vapor pressure.

7
Q

A solution is made up of mixing 3.81 g of naphthalene (C10H8 ) and 87.5 g benzene (C6H6 ). What is the vapor pressure of benzene above the solution?
Vapor pressure of benzene (P°solvent ) is 0.1252 atm.
The molecular weight of naphthalene is 128.17 g / mol and the molecular weight of benzene is 78.0 g / mol.

A

0.122 atm

8
Q

Which statement about colligative properties is not correct?

A

Two solutions with slightly different mole fractions for their solutes can still have the same value for a colligative property.

9
Q

Look at the two plots showing the deviations in Raoult’s law.

Which statement about these plots is not correct?

A

The plot indicates that, at any solute concentration, the vapor pressure of the endothermic solution reaction is always greater than the other two vapor pressure values for the other types of solutions.

10
Q

Which of the following statements about vapor pressure is not correct?

A

A solution that contains a nonvolatile solute will always have a higher vapor pressure than the solvent in pure form (i.e., the pure solvent).

11
Q

An aqueous solution of glucose, in which the mole fraction of glucose solute is 0.01, has the same vapor pressure, boiling point, and freezing point as an aqueous solution of sucrose, in which the mole fraction of sucrose solute is also 0.01. Which statement best describes the properties of these two solutions?

A

The two solutions have colligative properties. This is because the three effects mentioned (vapor pressure, boiling point, freezing point) depend exclusively on the number of solute particles in solution.

12
Q

What is a colligative property?

A

a property that only depends on the number of solute particles present in a given amount of solution

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