Which of the following statements is correct?a)Solutes that dissociate in water have molar mass higher than the molar mass of the solute calculated theoreticallyb)Solutes that associate in water have molar mass higher than the molar mass of the solute calculated theoreticallyc)Solutes that dissociate in water experience a decrease in colligative propertiesd)Colligative properties are independent of the number of particles of the solute in the solutionCorrect answer is option 'B'. Can you explain this answer?

Question

Answer

Solutes that associate in water have molar mass higher than the molar mass of the solute calculated theoretically

Explanation:
When solutes associate in water, they form aggregates or clusters due to intermolecular forces such as hydrogen bonding. This leads to a decrease in the number of particles in the solution compared to what would be expected based on the molar mass of the solute calculated theoretically. As a result, the molar mass of the solute appears to be higher than the calculated molar mass.

Colligative properties:
Colligative properties are properties of a solution that depend on the number of particles present, rather than the chemical identity of the solute. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

Effects of dissociation and association on colligative properties:
- When solutes dissociate in water, they break apart into individual ions or molecules. This results in an increase in the number of particles in the solution, leading to an increase in colligative properties.
- On the other hand, when solutes associate in water, they form larger aggregates or clusters. This decreases the number of particles in the solution, resulting in a decrease in colligative properties.

Example:
Let's consider an example of a solute that associates in water, such as acetic acid (CH3COOH). In its pure form, acetic acid exists as individual molecules. However, in water, it can associate through hydrogen bonding to form clusters. Each cluster consists of multiple acetic acid molecules.

If we were to calculate the molar mass of acetic acid theoretically, it would be the sum of the atomic masses of its constituent elements (carbon, hydrogen, and oxygen). However, when acetic acid associates in water, the presence of clusters reduces the effective number of particles in the solution. As a result, the observed colligative properties such as boiling point elevation or freezing point depression would indicate a higher molar mass than the calculated theoretical value.

Therefore, the correct statement is option B: Solutes that associate in water have a molar mass higher than the molar mass of the solute calculated theoretically.

Answer

On association of solute molecules, the number of particles in a solution is reduced. Since, colligative properties are directly proportional to the number of particles in a solution, the colligative properties reduce. The molecular mass of the solute is inversely proportional to its colligative properties. Therefore, solutes that associate in water have a higher molar mass than its value calculated theoretically.

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