Problem 12 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Determination of the molar mass of acetic acid in benzene using freezing point depression is affected by : (a) partial ionization (b) dissociation (c) complex formation (d) association

Step-by-Step Solution

Answer

Association affects the determination of acetic acid's molar mass in benzene through freezing point depression.

Step 1: Understand Freezing Point Depression

Freezing point depression is a colligative property that depends on the number of solute particles in a solution, not on the identity of those particles. When a solute is added to a solvent, the freezing point of the solution is lower than that of the pure solvent.

Step 2: Consider Possible Effects

Examine the options given: ionization, dissociation, complex formation, and association. These processes can change the number of particles (or effective particles) in the solution, thus affecting the freezing point depression observed.

Step 3: Analyze Each Option

- Partial ionization would increase the number of solute particles if the solute were ionic, but acetic acid in benzene is not ionic. - Dissociation would increase the number of solute particles if the molecules split into smaller entities. - Complex formation typically reduces the number of particles if multiple entities form a larger complex. - Association decreases the number of solute particles when molecules group together.

Step 4: Apply to Acetic Acid in Benzene

Acetic acid tends to associate in nonpolar solvents like benzene due to hydrogen bonding, leading to the formation of dimers. This reduces the number of moles of particles in the solution, thus lowering the degree of freezing point depression observed and affecting the calculation of the molar mass.

Step 5: Conclusion

The reduction in particle count due to association will lead to a higher measured molar mass than the actual value when calculated from the freezing point depression data.

Key Concepts

Colligative Properties

Colligative properties are characteristics of solutions that depend on the number of solute particles rather than their identity. This means that whether you are adding sugar or salt doesn't affect these properties directly, only the number of particles plays a role.
These properties include:

• Freezing point depression
• Boiling point elevation
• Osmotic pressure
• Vapor pressure lowering

A practical example of a colligative property is freezing point depression. When a solute is dissolved in a solvent, the solution's freezing point is lowered compared to that of the pure solvent. This occurs because solute particles disrupt the solvent's ability to organize into a solid structure, requiring lower temperatures for freezing. It is crucial to grasp these principles when studying phenomena like the freezing point depression that helps determine properties such as molar mass. More particles in the solution generally mean a greater change in these colligative properties.

Molar Mass Calculation

Calculating the molar mass using freezing point depression involves understanding the relationship between the change in freezing point and the properties of the solute. When a solute is added to a solvent, the amount by which the freezing point is lowered can be mathematically related to the concentration of solute particles in the solution.
The formula is straightforward:

• The freezing point depression ( delta T_f = T_f^0 - T_f , where T_f^0 is the freezing point of the pure solvent and T_f is the freezing point of the solution).
• The formula to find molar mass ( M_m = K_f imes w / ( delta T_f imes m ) , where K_f is the freezing point depression constant, w is the mass of the solute, and m is the mass of the solvent).

The relationship between these values allows for the determination of the molar mass of the solute, provided it behaves ideally. However, calculations can be complicated by factors like association or dissociation of solute particles, which must be considered to ensure accuracy.

Acetic Acid Association

Association refers to the process by which molecules come together to form larger entities. In the case of acetic acid in benzene, acetic acid molecules tend to form dimers. This happens due to hydrogen bonding between the acetic acid molecules, which is significant in nonpolar solvents such as benzene. The attraction between the molecules leads them to pair up rather than remain isolated.
When acetic acid associates into dimers:

• The effective number of solute particles decreases.
• This results in a smaller depression of the freezing point than expected from the concentration of acetic acid added.
• The observed molar mass appears higher than the true molar mass because there are fewer particles contributing to the colligative property effect.

Understanding this association is vital when interpreting results from experiments involving acetic acid and similar compounds to ensure accurate calculations.

Dimer Formation

Dimer formation is a specific type of molecular association. In this process, two identical molecules interact and link up to form a dimer, which is a unit consisting of these two bonded molecules. For acetic acid in benzene, dimer formation is prevalent due to strong hydrogen bonding between the carboxylic groups in acetic acid.
Important points about dimer formation include:

• Dimers reduce the number of independent particles in solution, affecting colligative properties.
• When dimers form, the measured molar mass as calculated from the experiment is artificially high.
• It's critical to account for dimerization when using freezing point depression data to compute molar mass, especially in nonpolar solvents where such associations are more likely.

Recognizing and accounting for dimer formation ensures the accuracy in the determination of molecular weight and understanding of the solution's behavior.

Nonpolar Solvents

Nonpolar solvents, like benzene, provide an interesting environment for molecular interactions. Unlike polar solvents, nonpolar solvents do not have significant charge differences within the molecule, so they do not easily dissolve ionic compounds. Instead, they interact primarily through Van der Waals forces and sometimes hydrogen bonding depending on the solutes.
In the context of acetic acid:

• Nonpolar solvents encourage association and dimerization of acetic acid through hydrogen bonds.
• These interactions are less about the solvent itself and more about how it doesn’t disrupt the attraction between polar groups of the acetic acid.
• This often results in reduced observable particles, impacting colligative property measurements like freezing point depression.

Considering the characteristics of nonpolar solvents is important for predicting and interpreting solute behavior, especially in chemical reactions and processes involving dissolved molecular compounds such as acetic acid.