How does dissolving a solute affect the boiling point or freezing point of the solution? What about the solution's vapor pressure?

Read about Raoult's Law, freezing point depression and boiling point elevation.

When a solute is dissolved in a solvent, it affects the boiling point, freezing point, and vapor pressure of the resulting solution. Raoult's Law, as well as freezing point depression and boiling point elevation, are concepts that help explain these changes.

Let's start with boiling point elevation. When a solute is dissolved in a solvent, the boiling point of the solution increases compared to the pure solvent. This happens because the solute particles disrupt the attractive forces between the solvent molecules, making it more difficult for them to escape the liquid phase and enter the gas phase. As a result, the solution needs to be heated to a higher temperature before it reaches its boiling point.

To calculate the boiling point elevation, you can use the equation:

ΔTb = i * K * m

Where:
ΔTb is the change in boiling point
i is the van't Hoff factor, which represents the number of particles the solute breaks into when dissolved (for example, NaCl breaks into two ions, so i would be 2)
K is the molal boiling point elevation constant, which depends on the solvent
m is the molality of the solution, measured in moles of solute per kilogram of solvent.

Moving on to freezing point depression. When a solute is dissolved in a solvent, the freezing point of the solution decreases compared to the pure solvent. The presence of solute particles interferes with the formation of the regular crystal lattice structure of the solvent during freezing, preventing it from solidifying at its normal freezing point. Thus, a lower temperature is required to freeze the solution.

The freezing point depression can be calculated using the equation:

ΔTf = i * K * m

Where:
ΔTf is the change in freezing point
i is the van't Hoff factor
K is the molal freezing point depression constant, which depends on the solvent
m is the molality of the solution.

Lastly, let's discuss vapor pressure. When a solute is dissolved in a solvent, the vapor pressure of the resulting solution is lower than that of the pure solvent. This is because the solute particles occupy space on the surface of the solvent, reducing the number of solvent molecules able to escape into the gas phase. As a result, it takes longer for the solution to evaporate and the vapor pressure is decreased.

Raoult's Law describes the relationship between vapor pressure and the composition of a solution. According to Raoult's Law, the vapor pressure of a component in a solution is proportional to its mole fraction in the solution. In other words, the vapor pressure is directly influenced by the amount of solute dissolved in the solvent.

In summary, dissolving a solute in a solvent affects the boiling point, freezing point, and vapor pressure of the resulting solution. The boiling point increases, the freezing point decreases, and the vapor pressure decreases. These changes can be quantitatively calculated using the equations provided, which consider factors such as the van't Hoff factor and the molal constants for boiling point elevation and freezing point depression.