5A3 Colligative Properties of Solutions

Question 1

Define:

colligative property

Answer 1

A property of a solution that depends on the ratio of solute to solvent particles.

Colligative properties are independent of the identity of the solute and solvent.

Question 2

List four colligative properties.

Answer 2

1. Boiling point elevation
2. Freezing point depression
3. Vapor pressure lowering
4. Osmotic pressure

Question 3

True or False:

Density is a colligative property.

Answer 3

False

Density is not dependent on the ratio of solute to solvent, unlike colligative properties.

Question 4

Define

van’t Hoff factor (i)

Answer 4

The number of particles the solute dissociates into in a solution.

For non-ionic solutes, ‘i’ is 1 because they do not dissociate into ions.

For ionic solutes, i is greater than 1, depending on the number of ions formed.

Question 5

What is the van’t Hoff factor for NaCl?

Answer 5

2

NaCl dissociates into two ions: Na⁺ and Cl⁻, so the van’t Hoff factor is 2.

Question 6

How does the van’t Hoff factor affect colligative properties?

Answer 6

It multiplies the effect of solute concentration.

Question 7

Define:

boiling point elevation

Answer 7

The increase in the boiling point of a solution compared to the pure solvent.

Boiling point elevation occurs when a solute is added to a solvent, requiring more energy to boil the solution. For example, adding salt to water when cooking will increase the boiling point, allowing for food to be cooked faster at a higher temperature.

Question 8

Fill in the blank:

Boiling point elevation is directly related to the _______ of the solution.

Answer 8

molality

(m)

The change in boiling point is directly proportional to the molality of the solution.

Question 9

What is the equation for boiling point elevation?

Answer 9

ΔTb = i * Kb * m

ΔTb is the change in boiling point, Kb is the boiling point elevation constant, and m is the molality.

Question 10

What happens to the boiling point of water when sugar is added?

Answer 10

The boiling point increases.

This is an example of boiling point elevation, a colligative property.

Question 11

Define:

osmotic pressure

Answer 11

The pressure required to stop the flow of solvent through a selectively permeable membrane from a lower concentration solution to a higher concentration solution.

It is important for cell function and is influenced by solute concentration.

Question 12

What happens to blood cells in a hypertonic solution?

Answer 12

They shrink due to the loss of water.

Osmotic pressure causes water to move out of the cells into the surrounding solution, leading to cell shrinkage.

A hypertonic solution is a type of solution that has a higher concentration of solutes on the outside of a cell than on the inside of the cell.

Question 13

How do you calculate osmotic pressure?

Answer 13

Osmotic pressure = i * m * R * T

where:
i is the van’t Hoff factor
m is the molarity
R is the ideal gas constant
T is temperature

Question 14

True or False:

Osmotic pressure increases with the concentration of solute.

Answer 14

True

Higher solute concentration results in a higher osmotic pressure, which drives the process of osmosis.

Osmosis continues until equilibrium is reached.

Question 15

What is the ideal gas constant (R) in osmotic pressure calculations?

Answer 15

0.0821 L·atm/(mol·K)

This is the value of the ideal gas constant used in the osmotic pressure formula.

Question 16

What does Raoult’s Law describe?

Answer 16

The relationship between the vapor pressure of a solution and the mole fraction of its components.

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

Question 17

List the components of Raoult’s Law equation.

Answer 17

• P (vapor pressure of solution)
• p (vapor pressure of pure solvent)
• x (mole fraction of solvent)

Raoult’s Law allows calculation of the vapor pressure of a solution based on the mole fraction of the solvent and the pure solvent’s vapor pressure.

Question 18

What is the equation for Raoult’s Law?

Answer 18

P=p_aX_a+p_bX_b…

where:
P is the vapor pressure of the solution
pa and pb are the vapor pressures of the pure solvents a and b
Xa and Xb are the mole fractions of a and b

Question 19

Define:

vapor pressure

Answer 19

The pressure exerted by the gas above a solution on its surface when the system is at equilibrium.

A high vapor pressure indicates that a solution will boil more readily.

Question 20

What happens to vapor pressure when a solute is added to a solution?

Answer 20

It decreases.

Solute particles occupy space at the surface, reducing solvent molecules’ escape rate.

Question 21

Fill in the blank:

A solution with a lower vapor pressure will boil at a _______ temperature.

Answer 21

higher

Lower vapor pressure means the solution will need a higher temperature to reach the boiling point.

Question 22

Define:

freezing point

Answer 22

The temperature at which a substance changes from the liquid to the solid phase.

The freezing point depends on the substance and its state, either as a pure substance or a solution.

Question 23

When happens to the freezing point of a solvent when a solute is added?

Answer 23

It is lowered.

The addition of solute makes it harder for the solvent molecules to form a lattice structure, requiring more energy to freeze, thus lowering the freezing point.

Question 24

List the factors that determine the change in freezing point.

Answer 24

• Molality (m)
• Freezing point depression constant (kf)
• van’t Hoff factor (i)

These factors are incorporated into the freezing point depression formula to calculate the temperature change in the solvent’s freezing point.

Question 25

Define:

freezing point depression

Answer 25

It is the decrease in the freezing point of a solvent due to the presence of solute particles.

This is a key concept in colligative properties.

Question 26

Define:

freezing point depression constant

(Kf)

Answer 26

An experimentally determined constant that represents how much the freezing point of a solvent is depressed per molal concentration of solute.

The more negative the Kf value, the more readily the solvent’s freezing point is depressed.

Question 27

True or False:

The freezing point depression constant (Kf) is always positive.

Answer 27

False

Kf is always negative, as freezing point depression lowers the freezing point of the solvent.

Question 28

What is the value of Kf for water?

Answer 28

1.86 °C/m

Kf is specific to each solvent and must be looked up for calculations involving freezing point depression.

Question 29

What does the symbol ‘i’ represent in the freezing point depression equation?

Answer 29

The number of molecules or ions the solute splits into when it dissolves.

‘i’ accounts for how a solute dissociates into ions or molecules in solution.

Question 30

How do you calculate the change in freezing point (ΔTf)?

Answer 30

ΔTf = -i * Kf * m

This equation calculates the freezing point depression based on the solute’s dissociation, the solvent’s Kf, and the solution’s molality.

Question 31

Which property makes molality suitable for colligative properties studies?

Answer 31

It is independent of temperature changes.

Molality uses the weight of the solvent, not volume, which varies with temperature.

Colligative properties depend on solute concentration, which molality accurately represents under varying temperatures.

Question 32

What is the molality (m) of a solution containing 0.342 moles of NaCl in 0.100 kg of water?

Answer 32

3.42 m

This is calculated by dividing moles of NaCl by the mass of water in kilograms.

Question 33

Fill in the blank:

The freezing point of pure water is ____ °C.

Answer 33

0

The freezing point of pure water is 0°C, which is used as a reference in calculating the freezing point depression.

Question 34

List the steps to find the molar mass of an unknown substance using freezing point depression data.

Answer 34

• Calculate the freezing point depression from the data.
• Calculate molality.
• Calculate the number of moles of solute.
• Calculate molar mass.

These steps are based on manipulating the freezing point depression equation (ΔTf = -i * Kf * m) to solve for molar mass.

Question 35

What is the molar mass of an unknown non-ionic substance if 10 g of it is dissolved in 500 g of water, and the freezing point is -0.5°C?

Answer 35

74.1 g/mol

Use the formula ΔTf = -i * Kf * m to calculate the molality (where ΔTf = -0.5, i = 1, kf =1.86, and m is unknown).

From the molality, calculate the number of moles followed by the molar mass.

Question 36

How does salting roads in winter demonstrate a colligative property?

Answer 36

It lowers the freezing point of water.

This ensures water remains in liquid form preventing ice formation.

This demonstrates freezing point depression, a colligative property.

Question 37

Fill in the blank:

A solute that dissociates into multiple ions will have a _______ freezing point depression.

Answer 37

higher

More solute particles (ions or molecules) will cause a greater decrease in freezing point.

Question 38

List two real-life examples of freezing point depression.

Answer 38

1. Saltwater in oceans
2. Antifreeze in car radiators

Salt in seawater lowers the freezing point, preventing it from freezing at typical freshwater freezing temperatures.

Antifreeze is added to prevent engine damage.

Question 39

Which factor is not included in the formula for freezing point depression?

Answer 39

The identity of the solute.

Freezing point depression is independent of the chemical identity of the solute, focusing instead on the quantity of solute particles.

Question 40

How would you calculate the mass of solute needed to achieve a specific freezing point depression?

Answer 40

Rearrange the freezing point depression equation to solve for molality and then use the molality to find the required solute mass.

This method involves determining the desired freezing point change, then calculating the molality and corresponding solute mass.