What are Colligative Properties?

What are Colligative Properties? The word “colligative”, derived from the Latin word “colligates”, means “bound together.” 

Colligative properties do not depend on the solution component’s chemical nature. Several qualities convey the concentration of a solution, like molality, polarity, and normality. Therefore, these colligative properties can be linked. 

Different Types of Colligative Properties

A solution can exhibit four types of Colligative Properties:

• Osmotic pressure.
• Relative lowering of vapour pressure.
• Depression in freezing point.
• The Freezing Point Constant (Kf)
• Elevation in boiling point.

Osmotic Pressure

Osmotic pressure is defined as the pressure necessary to prevent water from diffusing across a barrier created via osmosis. In another way, it relates to how strongly the water would have to “push” through the barrier to disperse to the other side. The diffusion of water across a semi-permeable membrane is known as osmosis. As a result, in osmosis, the solutes are unable to flow through the membrane.

The following equation can be used to compute osmotic pressure:

π=CRT

Where,

π = this is the abbreviation for osmotic pressure.

C= Molar concentration of the solution (The number of atoms, ions, or molecules in a solute is measured in molar concentration.).

R= is the universal gas constant.

T= Temperature in degrees Kelvin.

Let us consider that the solution contains 2 grams of solute, and the molar mass of the solute is M2. The volume of the solution is V (in litres).

Hence, the molar concentration can now be expressed as:

C = (w2/M2) ÷ V = w2 ÷ (V × M2).

So, osmotic pressure is:

π = (w2RT) ÷ (M2V).

Hence, the above equation can be rearranged as:

M2 = (w2RT) ÷ (πV).

Relative Lowering of Vapour Pressure

In a pure solvent, vapour pressure is reduced when a non-volatile solute is dissolved in it. The surface contains both solute and solvent molecules when a non-volatile solute is added to the solvent. Therefore, the amount of surface covered by solvent molecules gets reduced eventually. 

So now, P is referred to as the solvent’s vapour pressure and Ps the solution’s vapour pressure. Then, the difference between P – Ps is the lowering of vapour pressure, and the ratio is the relative of the lowering of the vapour pressure. 

In 1886, François-Marie Raoult, a French chemist, established a relative lowering between vapour pressure and mole fraction. That relationship is referred to as Raoult’s law. The law specifically says that the relative lowering in vapour pressure of a dilute solution is equal to the mole fraction of the solution’s solute.

Depression in Freezing Point

When a certain solute is introduced to a solvent, the solvent’s freezing point is lowered. It must be a non-volatile solute. Example: 

1. When salt is added to water, it becomes saltier.
2. When water is mixed with alcohol. 

The freezing point of the resulting solution or combination is lower than that of a pure solvent. The molal concentration of the solution’s solute and the decrease in freezing point are directly proportional.

The equation expresses this decrease in the freezing point:

ΔTf = Kf × m.

Tf is the freezing point depression in this equation.

The Freezing Point Constant (Kf)

The concentration of the solute determines the freezing point depression. The concentration of a solution is measured by its molality, defined as:

Molality =  Moles of solutekilograms of solvent

The molal concentration of the solution is denoted by the letter m. The amount of moles of solute per kg of a solvent is known as molality. However, we now understand that molality is determined by:

M = (1000 × w2) ÷ (w1 × M2).

In this scenario,

The molar mass of the solute is M2, and its weight is w2.

The solvent has a weight of w1.

Hence,

The term “freezing point depression” is defined as follows:

ΔTf = (Kf × 1000 × w2) ÷ (w1 × M2).

As a result, the equation becomes:

M2 = (Kf × 1000 × w2) ÷ (w1 × ΔTf).

The molecular weight of the solute is calculated in this way.

Elevation in Boiling Point

When a certain solute is introduced to a solvent, the solvent’s boiling point is raised. It must be a non-volatile solute. The molal concentration of the solution’s solute and the increase in boiling point are directly proportional.

Tb = Kbm = (1000 w2) (w1 M2). 

As a result, an increase in boiling point is indicated by:

ΔTb = (Kb × 1000 × w2) ÷ (w1 × M2).

As a result, the solute’s molecular weight becomes:

M2 = (Kb × 1000 × w2) ÷ (w1 × △Tb).

Conclusion

The significant factor of colligative properties is that they are dependent only on the solution’s number of solute particles present. The meaning of the definition of each colligative property is that they are precisely associated with each other. Therefore, if only one property of these colligative is measured, the other can similarly be calculated. These colligative properties of dilute solutions are highly significant. The properties provide useful methods for finding the weight of molecular weights and dissolved substances. In dilute solution, we mainly observe these colligative properties.