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Chemistry LibreTexts

Boiling Point Elevation

The colligative properties of a solution depend on the relative numbers (concentration) of solute and solvent particles, they do not depend on the nature of the particles. Colligative properties change in proportion to the concentration of the solute particles. We distinguish between four colligative properties: vapor pressure lowering, freezing point depression, boiling point elevation, and osmotic pressure. All four colligative properties fit the relationship

property = solute concentration x constant

Property Symbol Solute Concentration Proportionality Constant
Vapor pressure \(\Delta P\) mole fraction Po (vapor pressure of pure solvent)
Boiling Point \(\Delta{T_b}\) molal Kb (boiling point constant)
Freezing Point \(\Delta{T_f}\) molal Kf (freezing point constant)
Osmotic Pressure \(P\) molar RT

The determination of colligative properties allows us to determine the concentration of a solution and calculate molar masses of solutes

Boiling Point Elevation

The boiling points of solutions are all higher than that of the pure solvent. Difference between the boiling points of the pure solvent and the solution is proportional to the concentration of the solute particles:

\[\Delta{T_b} = T_b (solution) - T_b (solvent) = K_b \times m\]

where \(\Delta{T_b}\) is the boiling point elevation, \(K_b\) is the boiling point elevation constant, and m is the molality (mol/kg solvent) of the solute.

Exercise

A solution is prepared when 1.20 g of a compound is dissolved in 20.0 g of benzene. The boiling point of the solution is 80.94 oC.

  • What is the boiling point of pure benzene?
  • What is the molality of the solution?
  • What is the molar mass of the compound?

Answer

  • 1.8 x 102 g/mol)

 

Questions

  • Explain how a non-volatile solute lowers the vapor pressure, raises the boiling point and lowers the freezing point of a solvent.
  • Describe the process of osmosis and reverse osmosis
  • Explain why ocean water is not fit for consumption by humans