Quiz 1 - Colligative Properties of Solutions | Colligative Properties of Solutions

Molality (m) is defined as moles of solute per kilogram of solvent. Since it involves mass and not volume, it is independent of temperature. Colligative properties, which depend on the number of particles in solution and not on their identity, are appropriately discussed using molality as it remains constant with temperature changes.

Adding a non-volatile solute to a solvent decreases the vapor pressure of the solution (Raoult's law) because the solute particles occupy surface area and thus fewer solvent molecules can escape into the vapor phase. This principle underlies the vapor-pressure lowering of a colligative property.

The presence of a solute lowers the solvent's freezing point in a process known as freezing-point depression. This occurs because the solute particles disrupt the formation of the solvent's solid structure, requiring a lower temperature to solidify.

The Van't Hoff factor (i) accounts for the number of particles a compound dissociates into when dissolved in a solution. For strong electrolytes, which dissociate completely, the factor is used in calculations of colligative properties, such as osmotic pressure, boiling-point elevation, and freezing-point depression, to reflect the role of dissociated ions.

Osmotic pressure is a colligative property that is directly related to the concentration of solute particles in a solution. It can be used to determine the molar mass of macromolecules in solution by measuring the pressure needed to prevent osmosis.

The boiling point elevation is:
 

Since the solute is a nonelectrolyte i = 1.

ΔTb = 1 x 0.512 x 3 = 1.536 ^oC. The boiling point of water is 100 ^oC at sea level, so the expected boiling point of the solution will be 100 ^oC + ΔTb = 101.54 ^oC.

Both the depression in the freezing point and the elevation in boiling point are consequences of the vapor pressure lowering due to the addition of a non-volatile solute. The altered vapor pressure affects the temperatures at which the solvent transitions between the solid and liquid or liquid and gas phases.

Freezing-point depression depends on the number of solute particles in the solution and is independent of their size or charge. Hence, the effect is the same for large or small solute particles as long as the number of particles is the same.

CaCl₂ dissociates into 3 ions (one Ca²⁺ and two Cl^-), hence the Van't Hoff factor (i) is 3. The freezing-point depression is directly proportional to the product of the molal concentration and the Van't Hoff factor (ΔT_f = i K_f m). The other salts either dissociate into 2 particles only.

Osmotic pressure (Π) is directly proportional to the concentration of solute in a solution, so doubling the solute concentration doubles the osmotic pressure.