13.3.2 Boiling Point Elevation and Freezing Point Depression

Question 1

Boiling Point Elevation and Freezing Point Depression

Answer 1

• As solute is added to a solvent, the vapor pressure of the solvent decreases, increasing the boiling point of the solution.
• As more solute is added and the vapor pressure drops, the triple point of the solution decreases causing a depression in the freezing point.
• Mathematical relationships relating the concentration of solute to the change in freezing or boiling point are Tf = – Kfm and Tb = Kbm.

Question 2

note

Answer 2

• As solute is added to a solvent, the vapor pressure of the solvent decreases, increasing the boiling point of the solution. The normal boiling point is the temperature at which the liquid-vapor boundary crosses one atmosphere. According to Raoult’s law, when solute is added, the vapor pressure is lowered, raising the boiling point.
• This effect of solute on the boiling point of a solvent is known as boiling point elevation. Boiling point is a
  colligative property, depending on the amount of solute, not its identity.
• When the vapor pressure drops, the temperature at which the triple point occurs also drops, causing the solid-liquid boundary to shift to the left. This shift of the solid-liquid boundary results in a decreased freezing point.
• This effect of solute on the freezing point of a solvent is known as freezing point depression. Like boiling point elevation, it is also a colligative property.
• The mathematical expressions for the relationship between solute concentration and change in freezing and boiling point are shown to the left.
• The term ΔTb represents the change in the boiling point between the pure solvent and the solution.
• Kb (or Kf) represents a boiling (or freezing) point constant, which is specific to the solvent. The larger the constant, the more pronounced the effect on the boiling or freezing point.
• Concentration is in molality (m) (moles solute per kilogram solvent).

Question 3

How does depressing the vapor pressure affect the boiling point and the freezing point of a solution?

Answer 3

The vapor pressure raises the boiling point and lowers the freezing point.

Question 4

Which of the following is an ideal solution?

Answer 4

• A solution in which the solute has no vapor pressure.
• A solution in which the solute is non-volatile.
• A solution in which the solute obeys Raoult’s law.

Question 5

Colligative properties are properties of a solution that depend on the ________ but not the _________ of the solute particles.

Answer 5

number, identity

Question 6

Which of the following can be determined if the boiling-point elevation, the mass of solute and the molality of a given solution are known.

Answer 6

The molecular weight of the solute

Question 7

If the vapor pressure of a solution is directly proportional to the mole fraction of solvent present, the solution is said to be what?

Answer 7

ideal

Question 8

A student adds 0.10 mol of sodium chloride to a beaker containing 500 mL of water. The water is brought to a boil and the boiling point measured. The boiling point is elevated about twice as much as the student expected. What happened?

Answer 8

The student forgot that each unit of sodium chloride contributes two particles.

Question 9

Raoult’s law states that vapor pressure of a solvent ____ when a non-volatile solute is added.

Answer 9

decreases

Question 10

Adding a solute makes a solution more stable by increasing what?

Answer 10

Entropy

Question 11

How is the phase diagram of a solvent changed when a solute is added?

Answer 11

The vapor-liquid boundary moves down

Question 12

A colligative property depends on the…

Answer 12

amount of solute