How does the nature of the solute affect boiling point?

Answer 1

The solute raises the boiling point by an amount that depends on the number of particles it contributes go the solution.

Boiling point elevation is a colligative property. It depends only on the number of particles in the solution. The formula for boiling point elevation is

#ΔT_b = iK_b m#

where m is the molality of the solution, #K_b# is the molal boiling point elevation constant for the solvent, and i is a number related to the number of particles the solute contributes to the solution (the van’t Hoff i factor).

Non-electrolytes do not dissociate when they dissolve. Thus, one mole of glucose will provide one mole of particles, and i = 1.

#ΔT_b = K_b m#

NaCl dissociates into Na⁺ and Cl⁻ in water. So if you have 1 mol of NaCl, you have 2 mol of particles and i= 2. For CaCl₂, i = 3, for FeCl₃, i = 4, etc.

#ΔT_b = iK_b m#

EXAMPLE

Calculate the boiling point of a 0.15 m aqueous solution of sodium chloride. #K_b# for water is 0.512 °C•m⁻¹.

Solution

#ΔT_b = iK_b m# = 2 × 0.512 °C·m⁻¹ × 0.15 m = 0.15 °C

#T_b# = 100.00 °C + 0.15 °C = 100.15 °C

Here is a video of a lab examining this topic.

Here is a video which shows some sample calculations of boiling point elevation.

videos from: Noel Pauller

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Answer 2

The nature of the solute affects the boiling point by altering the vapor pressure of the solvent. Non-volatile solutes, such as salts or sugars, increase the boiling point of the solvent because they decrease the vapor pressure of the solution. This requires a higher temperature to reach the equilibrium vapor pressure and boil the solution.

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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