Which of the following colligative property is associated with concentration in terms of molarity?

Osmotic pressure

The correct answer is option 3. Osmotic pressure.

Colligative properties are properties of solutions that depend on the number of solute particles but not on the type of solute. These properties include:

i. Lowering of Vapor Pressure

ii. Elevation in Boiling Point

iii. Depression in Freezing Point

iv. Osmotic Pressure

Molarity vs. Molality

Molarity (M): The number of moles of solute per liter of solution. It’s dependent on the volume of the solution, which can change with temperature.

Molality (m): The number of moles of solute per kilogram of solvent. Unlike molarity, it’s independent of temperature since it’s based on mass.

Detailed Explanation of Each Colligative Property:

Lowering of Vapor Pressure

When a non-volatile solute is added to a solvent, the vapor pressure of the solvent decreases. This is because the solute particles occupy some space on the surface, reducing the number of solvent molecules that can escape into the vapor phase. This property is best described using molality because it depends on the relative amount of solute particles to solvent particles, which is independent of volume changes.

Elevation in Boiling Point

The boiling point of a solution is higher than that of the pure solvent. This happens because the presence of solute particles lowers the vapor pressure, requiring a higher temperature to reach the vapor pressure equal to the external pressure. This is also described using molality for the same reasons as lowering of vapor pressure—it’s the ratio of solute particles to the solvent mass that matters.

Depression in Freezing Point

The freezing point of a solution is lower than that of the pure solvent. Solute particles disrupt the formation of a solid structure by the solvent, requiring a lower temperature to freeze. This property is described using molality because the freezing point depression depends on the number of solute particles per mass of solvent.

Osmotic Pressure

Osmotic pressure (\(\pi\)) is the pressure required to stop the flow of solvent into the solution through a semipermeable membrane. It occurs because the solvent naturally moves from a region of lower solute concentration to higher solute concentration. Osmotic pressure is given by the equation:

\(\pi = MRT\)

where:

\(M\) is the molarity (moles of solute per liter of solution),

\(R\) is the gas constant,

\(T\) is the absolute temperature in Kelvin.

The equation shows a direct relationship between osmotic pressure and molarity, meaning the osmotic pressure increases with an increase in the concentration of the solute in terms of molarity.

Why Osmotic Pressure is Associated with Molarity

Molarity is the relevant concentration unit for osmotic pressure because osmotic pressure depends on the number of solute particles in a given volume of solution. Since the volume of the solution is directly involved in osmotic pressure, molarity (which is based on volume) is the natural choice for expressing concentration in this context.

Summary

Lowering of vapor pressure, elevation in boiling point, and depression in freezing point are best described using molality because they depend on the ratio of solute particles to the solvent’s mass, which remains constant irrespective of temperature changes. Osmotic pressure depends directly on the molarity because it’s influenced by the number of solute particles in a given volume of solution. Therefore, osmotic pressure is the colligative property associated with molarity.

So, the correct answer is 3. Osmotic pressure.