The Colligative Properties: Videos & Practice Problems

Colligative properties describe how a pure solvent transforms into a solution upon solute addition. Boiling point and osmotic pressure increase, while freezing point and vapor pressure decrease. The van’t Hoff factor (I) indicates the number of ions produced by solutes: ionic compounds dissociate into multiple ions, while covalent compounds remain intact, yielding I = 1. For osmolarity and osmolality, the formulas are: $\mathrm{Osmolarity}=IM$ and $\mathrm{Osmolality}=Im$.

The Colligative Properties Concept 1

Colligative properties describe how the addition of a solute to a pure solvent affects the solution's characteristics. When a solute is introduced, the solution undergoes changes in specific properties: boiling point and osmotic pressure increase, while freezing point and vapor pressure decrease. Understanding these changes is crucial for grasping the behavior of solutions.

The boiling point is defined as the temperature at which a liquid's vapor pressure equals the external pressure, leading to a state of equilibrium between the liquid and gas phases. As solute concentration rises, the boiling point elevates due to the solute particles disrupting the solvent's ability to vaporize.

In contrast, the freezing point is the temperature at which a solid and liquid coexist in equilibrium. When solute is added, the freezing point decreases, a phenomenon known as freezing point depression. This occurs because solute particles interfere with the formation of the solid structure of the solvent.

Vapor pressure refers to the pressure exerted by a vapor in equilibrium with its liquid. The addition of solute lowers the vapor pressure of the solvent, as solute particles occupy space at the liquid's surface, reducing the number of solvent molecules that can escape into the vapor phase.

Osmotic pressure is the pressure required to stop the flow of solvent into a solution through a semipermeable membrane, driven by the movement of water from an area of low solute concentration to an area of high solute concentration. This property increases with the addition of solute, as the concentration gradient becomes steeper.

In summary, the transition from a pure solvent to a solution involves significant changes in boiling point, freezing point, vapor pressure, and osmotic pressure, all of which are essential for understanding solution behavior in various scientific and practical applications.

The Colligative Properties Example 1

When a pure solvent, such as benzene (C₆H₆), has a solute like glucose added, the boiling point of the solution increases. This phenomenon is known as boiling point elevation, which occurs because the presence of solute particles disrupts the ability of solvent molecules to escape into the vapor phase, requiring a higher temperature to reach the boiling point.Benzene has a boiling point of 80.1 degrees Celsius. When glucose is added, we can expect the boiling point to rise above this initial temperature. Among the options provided, the only reasonable new boiling point is 89.6 degrees Celsius. This aligns with the principle that adding solute increases the boiling point, while other options either suggest no change or a decrease, which contradicts the established behavior of solutions.In summary, the addition of glucose to benzene results in a boiling point elevation, leading to a new boiling point of 89.6 degrees Celsius, demonstrating the colligative property of boiling point elevation in solutions.

The Colligative Properties Concept 2

The van't Hoff factor, represented by the variable \( i \), quantifies the number of ions produced when a soluble solute dissolves in a solvent. Solutes are categorized as either ionic or covalent. Ionic compounds consist of a positive ion paired with a negative ion, where the positive ion can be a metal or an ammonium ion, and the negative ion is typically a nonmetal. For example, sodium hydroxide (NaOH) dissociates into two ions: \( \text{Na}^+ \) and \( \text{OH}^- \), resulting in a van't Hoff factor of \( i = 2 \). Ammonium carbonate (NH₄)₂CO₃ breaks down into two ammonium ions and one carbonate ion, yielding a total of three ions, thus \( i = 3 \). Aluminum sulfate (Al₂(SO₄)₃) dissociates into two aluminum ions and three sulfate ions, giving a total of five ions, so \( i = 5 \). In contrast, covalent compounds, which consist solely of nonmetals, do not dissociate into ions when dissolved. These compounds are classified as non-volatile and non-electrolytes. Examples include glucose, chlorine, methanol, and urea. Since covalent solutes do not produce ions, their van't Hoff factor is always \( i = 1 \). This reflects that they remain intact in solution without breaking into ions, thus maintaining their molecular form. In summary, understanding the distinction between ionic and covalent solutes is crucial for accurately determining the van't Hoff factor, which influences colligative properties in solutions. Ionic compounds yield higher values of \( i \) due to their ability to dissociate into multiple ions, while covalent compounds consistently have a value of \( i = 1 \) as they do not dissociate.

The Colligative Properties Example 2

To determine which compound has the largest Van't Hoff factor (i), we need to analyze how each compound dissociates in solution. The Van't Hoff factor represents the number of particles into which a compound dissociates in solution.

Starting with aluminum chloride (AlCl₃), this ionic compound dissociates into one aluminum ion (Al³⁺) and three chloride ions (Cl^-), resulting in a total of four ions. Therefore, the Van't Hoff factor (i) for aluminum chloride is 4.

Next, we consider a covalent compound, which consists solely of nonmetals. Such compounds do not dissociate into ions, so their Van't Hoff factor is 1.

For zinc oxide (ZnO), this ionic compound dissociates into one zinc ion (Zn²⁺) and one oxide ion (O^2-), leading to a total of two ions. Thus, the Van't Hoff factor for zinc oxide is 2.

Ammonia (NH₃) is also a covalent compound and does not dissociate into ions, giving it a Van't Hoff factor of 1. Similarly, diphosphorus pentasulfide (P₂S₅) is a covalent compound with a Van't Hoff factor of 1 as well.

In summary, the compound with the highest Van't Hoff factor is aluminum chloride (AlCl₃), with a value of 4, indicating it produces the most particles in solution compared to the other compounds analyzed.

The Colligative Properties Concept 3

Colligative properties are essential concepts in chemistry that describe how the addition of solute affects the physical properties of a solvent. When more solute is added, properties such as boiling point and osmotic pressure increase, while freezing point and vapor pressure decrease. The extent of these changes is influenced by the number of solute particles in solution, which is quantified using the van't Hoff factor (I). This factor represents the number of ions or particles that a solute dissociates into when dissolved.

To calculate the effects of solute on colligative properties, we can use two important concepts: osmolarity and osmolality. Osmolarity refers to the concentration of solute particles in a solution and is defined as:

Osmolarity (Osm) = I × M

where I is the van't Hoff factor and M is the molarity of the solution. Similarly, osmolality, which is the concentration of solute particles in a solvent, is given by:

Osmolality (Osm) = I × m

where m is the molality of the solution. Understanding these formulas allows us to determine the total concentration of solute particles, which is crucial for predicting which solutions will exhibit higher boiling points or lower freezing points.

In summary, by applying the concepts of osmolarity and osmolality, we can effectively analyze the impact of solute concentration on the colligative properties of solutions, aiding in various applications in chemistry and related fields.

The Colligative Properties Example 3

To determine the ionic molality of potassium ions in a 1.18 molal solution of potassium phosphate, we first need to understand the dissociation of potassium phosphate in solution. Potassium phosphate dissociates into three potassium ions (K⁺) and one phosphate ion (PO₄^3-). This means that for every formula unit of potassium phosphate, there are a total of four ions produced: three potassium ions and one phosphate ion.

The ionic molality can be calculated using the formula:

Ionic Molality = Number of Ions × Molality of the Compound

In this case, the number of potassium ions is 3, and the molality of the potassium phosphate solution is 1.18 molal. Therefore, we can calculate the ionic molality of potassium ions as follows:

Ionic Molality = 3 × 1.18 molal = 3.54 molal

Thus, the ionic molality of potassium ions in the solution is 3.54 moles per kilogram of solvent.

Which of the following compounds will have the highest boiling point?

• A. 0.10 M sucrose
• B. 0.10 M AgCl
• C. 0.25 M NH₄NO₃
• D. 0.45 M pure water

Answer: (C)

Which of the following compound will have the highest vapor pressure?

• A. 0.45 m dinitrogen pentoxide
• B. 0.10 m aluminum chloride
• C. 0.50 m iron (III) perchlorate
• D. 0.15 m calcium phosphate
• E. 0.30 m Potassium sulfide

Answer: (B)

Here’s what students ask on this topic:

What are the four colligative properties and how do they change with the addition of a solute?

The four colligative properties are boiling point, freezing point, vapor pressure, and osmotic pressure. When a solute is added to a pure solvent, the boiling point and osmotic pressure increase, while the freezing point and vapor pressure decrease. Boiling point is the temperature at which the liquid and gas phases are in equilibrium. Freezing point is the temperature at which the solid and liquid phases are in equilibrium. Vapor pressure is the pressure exerted by a gas at the surface of a liquid. Osmotic pressure is the force driving osmosis, the movement of water from an area of low concentration to high concentration.

What is the van't Hoff factor and how is it determined for ionic and covalent compounds?

The van't Hoff factor (I) represents the number of ions produced when a solute dissolves. For ionic compounds, it is determined by the number of ions the compound dissociates into. For example, NaOH dissociates into Na⁺ and OH⁻, so I = 2. For covalent compounds, which do not dissociate into ions, I is always 1. Examples of covalent compounds include glucose and urea, which remain intact in solution.

How do you calculate osmolarity and osmolality?

Osmolarity and osmolality are calculated using the van't Hoff factor (I) and the concentration of the solute. The formulas are:

$\mathrm{Osmolarity}=IM$

$\mathrm{Osmolality}=Im$

where M is molarity (moles of solute per liter of solution) and m is molality (moles of solute per kilogram of solvent). These calculations help determine the amount of solute in a solution, which affects colligative properties like boiling point and freezing point.

Why do boiling point and osmotic pressure increase with the addition of a solute?

Boiling point increases with the addition of a solute because the solute particles disrupt the solvent molecules, requiring more energy (higher temperature) to reach the boiling point. Osmotic pressure increases because the addition of solute creates a concentration gradient, causing water to move from an area of low solute concentration to high solute concentration, generating pressure. This movement of water is driven by osmosis, and the pressure needed to stop this flow is the osmotic pressure.

How does the addition of a solute affect the freezing point and vapor pressure of a solvent?

The addition of a solute lowers the freezing point of a solvent because the solute particles interfere with the formation of the solid phase, requiring a lower temperature to freeze. Vapor pressure decreases because the solute particles reduce the number of solvent molecules at the surface that can escape into the gas phase, thus lowering the pressure exerted by the vapor.