Question

Determination of the molar mass of acetic acid in benzene using freezing point depression is affected by :

• A. dissociation
• B. association
• C. partial ionization
• D. complex formation

Answer 1

The Correct Option is B

The determination of the molar mass of a solute using the freezing point depression method is based on colligative properties. In this particular case, the effect of the acetic acid (solute) on the freezing point of benzene (solvent) is used to determine the molar mass. The correct answer is "association". Let's explore why association affects the molar mass determination of acetic acid in benzene and rule out the other options:

1. Association: Acetic acid in benzene tends to form dimers due to hydrogen bonding. This association causes a decrease in the number of solute particles in the solution compared to what is expected for the same amount of acetic acid if it existed as individual molecules. When calculating the molar mass using the observed freezing point depression, the result will appear larger than the actual molar mass because the effective number of particles (or the van't Hoff factor) is reduced. Thus, this is the correct answer.
2. Dissociation: Dissociation involves the breaking apart of molecules into smaller units or ions. In non-polar solvents like benzene, acetic acid does not dissociate because benzene is not polar and does not support ionization. Therefore, dissociation is not relevant in this case.
3. Partial Ionization: This concept applies to ionic compounds or weak electrolytes in polar solvents. Acetic acid in benzene does not ionize because benzene is non-polar and does not facilitate ionization. Hence, partial ionization does not affect the determination in this scenario.
4. Complex Formation: Complex formation involves the creation of a compound composed of multiple different molecules or ions bonded together. Benzene doesn't facilitate such complex formation with acetic acid, as benzene is an aromatic hydrocarbon with limited reactivity towards acetic acid. Therefore, it does not affect the determination of molar mass in this context.

Given this analysis, the phenomenon of association due to dimer formation of acetic acid in benzene is the primary factor affecting the determination of molar mass using freezing point depression.