Boiling Point Elevation - Online Tutor, Practice Problems & Exam Prep

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

Here we're going to discuss the phenomenon when adding a solute to a pure solvent results in an increase in the boiling point of the solvent. Remember, the more solute you add, the higher your boiling point will be. Now, regarding this idea of boiling point, we have what's called the normal boiling point and then the boiling point of the solution that's created. We refer to the normal boiling point as bp, and this is the boiling point of the pure solvent before the addition of solute. Before any solute is added, this is basically the boiling point of the pure solvent. Now, the boiling point of the solution, which we will label as bp_solution, is the boiling point of the solvent after the addition of the solute. Remember, a solution is formed when we add solute to a solvent.

Looking at boiling point elevation, remember it increases as more solute is added. We have four areas that we need to focus on. The first area deals with the boiling point elevation formula. Here we express change in our boiling point as Δt_b=I∗k_b∗m. I is related to our Van't Hoff factor, and these are our variables. We define k_b as the boiling point constant of our pure solvent in units of degrees Celsius over molality, and m as the molality of the solution in moles of solute per kilogram of solvent. Once we've used this boiling point elevation formula and understand its different variables, we can then determine the boiling point of our solution. Here, we express the boiling point of the solution as the boiling point of our pure solvent—this is our normal boiling point—plus Δt_b.

The solvents that are customarily used in questions like this include water, benzene, chloroform, or ethanol. Here are their normal boiling points. If we add solute to them, we'd expect their boiling points to be higher than these that are reported here. Their k_b values, their boiling point constants, are these values respectively: 0.51, 2.53, 3.60, and 1.20. No, you don't need to memorize these numbers. These are just different types of solvents that you might encounter in a question dealing with colligative properties.

One last thing, recall if a compound is covalent, non-volatile, or non-ionic, meaning not an electrolyte, then its Van't Hoff factor is going to be equal to 1. So, just keep in mind when it comes to boiling point elevation, these are the key areas you need to pay attention to in order to find the boiling point of a solution.

Boiling Point Elevation Example 1

Calculate the boiling point of a 3.71 molal aqueous calcium bromide solution. Alright. So they want us to find our boiling point. So remember your boiling point of solution equals the boiling point of your pure solvent plus the change in the boiling point. So the boiling point of our pure solvent, what exactly is our solvent here? Well, here they're saying aqueous. When we say aqueous, that means the solvent is water. Pure water boils at 100 degrees Celsius. All we have to do now is figure out what our new change in boiling point will be. So we're going to say Δt_b=I∗k_b∗m. Remember, I is the number of ions your solute will dissolve into when placed in a solvent. Here, calcium bromide is ionic. It breaks up into a calcium ion plus 2 bromide ions for a total of 3 ions. The solvent is water. The boiling point constant of water is 0.51 degrees Celsius over molality. Then finally, we're going to say our molality is given to us as 3.71 molal. So molalities cancel out and I'll have my answer here as degrees Celsius, which comes out to 5.68 degrees Celsius. So we take that and we plug it here. So when we add those together, our new temperature is 105.68 degrees Celsius. Remember, it's called boiling point elevation. We go up as we add solute to our pure solvent. Here, pure water was at 100 degrees Celsius. After the addition of calcium bromide, it's now at 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

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

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

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

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.