Problem 61 - A Complete Resource Book in Chemistry for JEE Main

Question

Osmotic pressure observed when benzoic acid is dissolved in benzene is less than that expected from theoretical considerations. This is because (a) benzoic acid is an organic solute (b) benzoic acid has higher molar mass than benzene (c) benzoic acid gets associated in benzene (d) benzoic acid gets dissociated in benzene

Step-by-Step Solution

Answer

(c) benzoic acid gets associated in benzene.

Step 1: Understand Osmotic Pressure

Osmotic pressure is a colligative property, which means it depends on the number of solute particles present in a solution, not on the nature of the solute itself. The expectation is that more solute particles result in higher osmotic pressure.

Step 2: Consider Dissociation vs. Association

When a solute like benzoic acid is added to a solvent like benzene, it can either dissociate (breaking down into more particles) or associate (binding to form fewer particles). Dissociation increases, while association decreases the number of particles, thus affecting osmotic pressure.

Step 3: Analyzing the Compounds Involved

Benzoic acid in benzene tends to form dimers (two molecules binding together) due to hydrogen bonding (association), thereby reducing the total number of particles in the solution. Benzene, being non-polar, does not promote dissociation of benzoic acid.

Step 4: Relate to Observed Phenomenon

Since benzoic acid dimerizes in benzene, the effective number of solute particles decreases. This reduction in particle number results in observed osmotic pressure being lower than theoretical expectations, which assume individual particles.

Step 5: Choose the Correct Answer

Given the explanation, the lower osmotic pressure is due to the association of benzoic acid molecules in benzene, leading to option (c) as the correct choice.

Key Concepts

Osmotic Pressure

Osmotic pressure is a fascinating concept that plays a crucial role in understanding how solutions behave. It is one of the colligative properties, which are properties that depend on the number of solute particles in a solution rather than the nature of those particles. Osmotic pressure occurs when a semipermeable membrane separates a solution from pure solvent, and allows only the solvent molecules to pass through. The pressure that builds up on the solution side is the osmotic pressure. This pressure is vital because it helps us understand how substances move across cell membranes in biological systems.

When we talk about osmotic pressure, it helps to remember a few key points:

• The greater the concentration of solute particles, the higher the osmotic pressure. This is because more solute particles effectively "push" the solvent molecules across the membrane.
• In a solution where the solute associates or forms larger aggregates instead of existing as distinct particles, osmotic pressure decreases because there are fewer individual particles contributing to the pressure.
• When theoretical expectations do not match observations, such as in the case of benzoic acid in benzene, the explanation often lies in understanding solute associations or dissociations that affect particle count.

Understanding osmotic pressure is crucial for applications in biology, chemistry, and industry where the behavior of solutions influences processes.

Particle Association

Particle association is a key factor that impacts the properties of a solution, including osmotic pressure. Association refers to the ability of molecules to come together forming larger, complex structures. In the case of benzoic acid dissolved in benzene, molecules associate to form dimers.

Association occurs when molecules exhibit attractions, such as hydrogen bonding, which is strong in polar substances. Benzoic acid can form hydrogen bonds even in non-polar solvents, leading molecules to pair up. When two benzoic acid molecules form a dimer, the number of effective particles in the solution decreases. With fewer particles, the osmotic pressure also lowers than expected.

In studying particle association, consider:

• Hydrogen bonding: This is the primary force that can cause molecules to dimerize in non-polar solvents.
• Solvent characteristics: A non-polar solvent like benzene doesn't provide significant competition to hydrogen bonding, promoting association rather than dissociation.

Therefore, understanding particle association helps predict and explain unusual behaviors in solutions.

Solute-Solvent Interactions

Solute-solvent interactions are crucial in determining how a solute will behave in a given solvent. These interactions dictate whether a solute will remain as individual particles, dissociate into smaller parts, or associate into larger complexes.

In a system like benzoic acid in benzene, the nature of the solvent plays a critical role. Since benzene is a non-polar solvent, it does not strongly interact with polar solute molecules like benzoic acid. This lack of a strong solvent-solute interaction encourages benzoic acid molecules to form hydrogen-bonded dimers, rather than dispersing as individual molecules or ions.

The important aspects to consider with solute-solvent interactions include:

• Polar vs. Non-polar Solvents: Polar solvents will often promote more dissociation due to their capability to stabilize ions, whereas non-polar solvents, like benzene, will likely encourage association.
• Solute Properties: The intrinsic properties of the solute, such as its ability to hydrogen bond, greatly affect how it will interact with the solvent.

Understanding these interactions is key to predicting how solutions will behave under various conditions and contributes to phenomena like the reduced osmotic pressure observed in specific solvent-solute systems.