Boiling Point Elevation: Videos & Practice Problems

Adding a solute to a pure solvent elevates the boiling point of the solution, a phenomenon known as boiling point elevation. The formula for this change is $\mathrm{\Delta T}$_b=I⋅K_b⋅m, where I is the van't Hoff factor, K_b is the boiling point constant, and m is the molality. The boiling point of the solution equals the normal boiling point plus $\mathrm{\Delta T}$_b. Common solvents include water and benzene, each with specific boiling point constants.

Boiling Point Elevation Concept 1

When a solute is added to a pure solvent, the boiling point of the solvent increases, a phenomenon known as boiling point elevation. The more solute that is added, the higher the boiling point becomes. Understanding this concept involves distinguishing between the normal boiling point of the solvent and the boiling point of the resulting solution. The normal boiling point (denoted as bp) refers to the boiling point of the pure solvent before any solute is added, while the boiling point of the solution (denoted as bp solution) is the boiling point after the solute has been introduced.

The relationship between the amount of solute and the increase in boiling point can be quantified using the boiling point elevation formula:

\(\Delta t_b = I \cdot k_b \cdot m\)

In this equation, \(\Delta t_b\) represents the change in boiling point, \(I\) is the van 't Hoff factor, which indicates the number of particles the solute dissociates into, \(k_b\) is the boiling point elevation constant specific to the solvent (expressed in degrees Celsius per molality), and \(m\) is the molality of the solution, defined as moles of solute per kilogram of solvent.

To find the boiling point of the solution, the following relationship is used:

\(\text{bp solution} = \text{bp} + \Delta t_b\)

Common solvents used in boiling point elevation problems include water, benzene, chloroform, and ethanol, each with their respective normal boiling points and boiling point constants:

• Water: \(k_b = 0.51 \, \text{°C/m}\)
• Benzene: \(k_b = 2.53 \, \text{°C/m}\)
• Chloroform: \(k_b = 3.60 \, \text{°C/m}\)
• Ethanol: \(k_b = 1.20 \, \text{°C/m}\)

It is important to note that for covalent, non-volatile, or non-ionic compounds (which do not dissociate into ions), the van 't Hoff factor \(I\) is equal to 1. Understanding these key concepts and relationships is essential for calculating the boiling point of a solution accurately.

Boiling Point Elevation Example 1

To calculate the boiling point of a 3.71 molal aqueous calcium bromide solution, we start by identifying the boiling point of the pure solvent, which is water. The boiling point of pure water is 100 degrees Celsius.

The change in boiling point, denoted as ΔT_b, can be determined using the formula:

ΔT_b = I × K_b × m

In this equation:

• I is the van 't Hoff factor, representing the number of particles the solute dissociates into. For calcium bromide (CaBr₂), it dissociates into one calcium ion (Ca²⁺) and two bromide ions (Br^-), resulting in a total of three ions. Thus, I = 3.
• K_b is the boiling point elevation constant for water, which is 0.51 degrees Celsius per molal.
• m is the molality of the solution, given as 3.71 molal.

Now, substituting the values into the equation:

ΔT_b = 3 × 0.51 × 3.71

Calculating this gives:

ΔT_b = 5.68 degrees Celsius

To find the new boiling point of the solution, we add the change in boiling point to the boiling point of the pure solvent:

Boiling Point = 100 + 5.68 = 105.68 degrees Celsius

This phenomenon is known as boiling point elevation, where the boiling point of the solvent increases upon the addition of a solute. Therefore, the boiling point of the 3.71 molal aqueous calcium bromide solution is 105.68 degrees Celsius.

An ethylene glycol solution contains 25.2 g of ethylene glycol (C₂H₆O₂) in 99.5 mL of water. Determine the change in boiling point. Assume a density of 1.00 g/mL for water.

• A. 18.4°C
• B. 9.22°C
• C. 2.08°C
• D. 0.572°C

Answer: (C)

Pure water boils at 100ºC. What is the new boiling point of water after the addition of 13.12 g aluminum chloride, AlCl₃, to 615 g water?

• A. 100.25°C
• B. 100.08°C
• C. 100.33°C
• D. 100.16°C

Answer: (C)

What is the molality of glucose in an aqueous solution if the boiling point of the solution is 103.15ºC?

• A. 0.15 m
• B. 0.29 m
• C. 6.18 m
• D. 1.6 m

Answer: (C)

Carbon dioxide is dissolved in 722 mL of benzene with a density of 1.59 g/mL. What mass of carbon dioxide would you add to make the boiling point of the solution 104.7ºC?

• A. 11 g
• B. 500 g
• C. 22.9 g
• D. 491 g

Answer: (D)

Here’s what students ask on this topic:

What is boiling point elevation and how does it occur?

Boiling point elevation is the phenomenon where the boiling point of a solvent increases when a solute is added. This occurs because the solute particles disrupt the solvent molecules, making it harder for them to escape into the gas phase. As a result, more heat (higher temperature) is required to reach the boiling point. The change in boiling point (ΔTb) can be calculated using the formula:

$\mathrm{\Delta T}{b}_{}=I\cdot K_b\cdot m$

where I is the van't Hoff factor, Kb is the boiling point constant, and m is the molality of the solution.

How do you calculate the boiling point of a solution?

To calculate the boiling point of a solution, you first need to determine the change in boiling point (ΔTb) using the formula:

$\mathrm{\Delta T}{b}_{}=I\cdot K_b\cdot m$

Here, I is the van't Hoff factor, Kb is the boiling point constant, and m is the molality. Once you have ΔTb, add it to the normal boiling point of the pure solvent:

$T_b=T_b+\mathrm{\Delta T}{b}_{}$

This gives you the boiling point of the solution.

What is the van't Hoff factor and how does it affect boiling point elevation?

The van't Hoff factor (I) represents the number of particles into which a solute dissociates in solution. For example, NaCl dissociates into two ions (Na⁺ and Cl⁻), so its van't Hoff factor is 2. The van't Hoff factor affects boiling point elevation because it determines the number of solute particles in the solution, which in turn affects the extent to which the boiling point is elevated. The formula for boiling point elevation is:

$\mathrm{\Delta T}{b}_{}=I\cdot K_b\cdot m$

where a higher I value results in a greater ΔTb.

What are some common solvents used in boiling point elevation problems?

Common solvents used in boiling point elevation problems include water, benzene, chloroform, and ethanol. Each of these solvents has a specific boiling point constant (Kb) that is used in the boiling point elevation formula:

$\mathrm{\Delta T}{b}_{}=I\cdot K_b\cdot m$

For example, the Kb values for these solvents are approximately 0.51 °C·kg/mol for water, 2.53 °C·kg/mol for benzene, 3.60 °C·kg/mol for chloroform, and 1.20 °C·kg/mol for ethanol. These constants are used to calculate the change in boiling point when a solute is added.

How does molality affect boiling point elevation?

Molality (m) is a measure of the concentration of a solute in a solution, defined as the number of moles of solute per kilogram of solvent. It directly affects boiling point elevation because it is one of the variables in the formula:

$\mathrm{\Delta T}{b}_{}=I\cdot K_b\cdot m$

As molality increases, the number of solute particles in the solution increases, leading to a greater disruption of the solvent molecules and a higher boiling point elevation. Therefore, a higher molality results in a larger ΔTb.