Question

Which one of the following is not a colligative property? A. Boiling point elevation B. Vapor pressure of a mixture C. Osmotic pressure D. Entropy of dissolution

Short answer

D. Entropy of dissolution

Step-by-step solution

Define Colligative Properties

Colligative properties are properties of solutions that depend on the number of solute particles present but not on their nature. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Analyze Each Option

Evaluate each given option to determine if it is a colligative property:A. Boiling point elevation: This is a colligative property because it depends on the number of solute particles.B. Vapor pressure of a mixture: Vapor pressure lowering is a colligative property; however, the vapor pressure itself is not solely dependent on the number of solute particles.C. Osmotic pressure: This is a colligative property because it depends on the number of solute particles in the solution.D. Entropy of dissolution: Entropy of dissolution is related to the disorder of the system and does not solely depend on the number of solite particles, hence it is not a colligative property.

Identify the Correct Answer

Based on the analysis, options A, B, and C are colligative properties. Option D, Entropy of dissolution, is not a colligative property.

Key concepts

Boiling Point Elevation

Boiling point elevation is a key colligative property of solutions. When a solute is added to a solvent, the boiling point of the solution increases. This happens because the solute particles disrupt the formation of vapor bubbles within the liquid, requiring more heat energy to allow the solvent particles to escape into the gas phase. The extent of boiling point elevation is directly proportional to the number of solute particles and can be expressed using the formula: \[\text{Δ}T_b = iK_b m \]
where \(\text{Δ}T_b\) is the boiling point elevation, \(i\) is the van 't Hoff factor (number of particles the solute dissociates into), \(K_b\) is the ebullioscopic constant of the solvent, and \(m\) is the molality of the solution.

Vapor Pressure Lowering

Vapor pressure lowering is another important colligative property. It occurs when a non-volatile solute is dissolved in a solvent, which results in a reduction of the solvent's vapor pressure. This is because the presence of solute particles at the liquid's surface decreases the number of solvent molecules that can escape into the vapor phase. The relationship between the vapor pressure of the solution and the pure solvent is described by Raoult's law: \[\text{P}_{\text{solution}} = \text{P}_{\text{solvent}} \times X_{\text{solvent}} \]
Here, \( \text{P}_{\text{solution}} \) is the vapor pressure of the solution, \( \text{P}_{\text{solvent}} \) is the vapor pressure of the pure solvent, and \( X_{\text{solvent}} \) is the mole fraction of the solvent. This property depends solely on the number of solute particles, not their nature.

Osmotic Pressure

Osmotic pressure is a colligative property that arises when a solvent passes through a semipermeable membrane to balance solute concentrations on both sides of the membrane. The osmotic pressure of a solution can be calculated using the formula: \[\text{Π} = iMRT \]
where \( \text{Π} \) is the osmotic pressure, \(i\) is the van 't Hoff factor, \(M\) is the molarity of the solution, \(R\) is the gas constant, and \(T\) is the temperature in Kelvin. Osmotic pressure is crucial in many biological processes, such as the regulation of water and nutrient flow in and out of cells. It highlights how the number of solute particles, rather than their specific type, influences the property.

Entropy of Dissolution

Entropy of dissolution is not a colligative property but an essential concept in understanding solution behavior. When a solute dissolves in a solvent, the system's disorder, or entropy, typically increases because the solute particles are more randomly distributed throughout the solvent. This change in entropy is a driving force in the dissolution process and depends on both the nature of the solute and solvent, as well as the interactions between them. Unlike colligative properties, which depend only on the number of solute particles, entropy of dissolution is related to the specific characteristics of the substances involved and their resultant interactions. This key distinction explains why entropy of dissolution is not classified as a colligative property.